fix: drop yarn in favour of npm per upstream

In signalapp/Signal-Desktop@2478bf1b the upstream
moved from using yarn to using npm for their build
tooling.

This commit updates the snapcraft.yaml to follow suit.

snap/snapcraft.yaml

@@ -76,23 +76,18 @@ parts:
       LINUX_CI_STEPS="$(curl -s "${UPSTREAM_URL}/.github/workflows/ci.yml")"
 
       NODE_VERSION="$(curl "${UPSTREAM_URL}/package.json" | jq -r '.engines.node')"
-      YARN_VERSION="$(echo "$LINUX_CI_STEPS" | grep -m1 -Po "npm install -g yarn@\K[^ ]+")"
-      NPM_VERSION="$(echo "$LINUX_CI_STEPS" | grep -m1 -Po "npm install -g yarn@[\d.]+ npm@\K[^ ]+")"
+      NPM_VERSION="$(echo "$LINUX_CI_STEPS" | grep -m1 -Po "npm install -g npm@\K[^ ]+")"
 
       # Install the correct version of nodejs required by Signal-Desktop
       asdf plugin add nodejs
       asdf install nodejs "$NODE_VERSION"
       asdf global nodejs "$NODE_VERSION"
 
       # Configure NPM proxy
-      npm config set proxy "${http_proxy:-}"
-      npm config set https-proxy "${https_proxy:-}"
-
-      # Install and configure Yarn
-      npm install -g "yarn@${YARN_VERSION}" "npm@${NPM_VERSION}"
-      yarn config set python /usr/bin/python3
-      yarn config set proxy "${http_proxy:-}"
-      yarn config set https-proxy "${https_proxy:-}"
+      npm config set -g registry https://registry.npmjs.org/
+      npm config set -g proxy "${http_proxy:-}"
+      npm config set -g https-proxy "${http_proxy:-}"
+      npm install -g "npm@${NPM_VERSION}"
 
   signal-desktop:
     after:
@@ -111,25 +106,22 @@ parts:
     build-environment:
       - SIGNAL_ENV: "production"
     override-build: |
-      # Use the version of nodejs/yarn we configured before
+      # Use the version of nodejs we configured before
       source "$(pwd)/../../nodejs/build/asdf.sh"
 
       git lfs install
 
-      # Disable yarn auto clean functionality
-      rm .yarnclean
-
       # If we're in a proxy environment, we need to patch some packages
       if [[ -n "${http_proxy:-}" ]]; then
         # Setup proxy access
         export HTTP_PROXY="${http_proxy}"
-        export HTTPS_PROXY="${https_proxy}"
+        export HTTPS_PROXY="${http_proxy}"
         export ELECTRON_GET_USE_PROXY=1
         export GLOBAL_AGENT_HTTP_PROXY="${http_proxy}"
         export GLOBAL_AGENT_HTTPS_PROXY="${http_proxy}"
 
         # The patch applied uses this package to ensure proxy is used
-        yarn global add https-proxy-agent
+        npm install https-proxy-agent
         
         # Update the package.json so the build uses the patched libraries
         cat package.json \
@@ -139,30 +131,30 @@ parts:
 
       # Build the sticker-creator
       pushd sticker-creator
-      yarn install
-      yarn build
+      npm ci --legacy-peer-deps
+      npm run build
       popd
 
       # Install the dependencies for the Signal-Desktop application.
-      # We cannot use `--frozen-lockfile` due to the patching above.
-      yarn install
+      npm install
+      npm ci --legacy-peer-deps
 
-      # This is the equivalent of 'yarn generate'. The upstream package.json
+      # This is the equivalent of 'npm run generate'. The upstream package.json
       # uses npm-run-all to run multiple things, which has a bug that prevents
       # this from succeeding in the snapcraft build env, so this just breaks
       # that apart into its component parts.
-      yarn build-protobuf
-      yarn build:esbuild
-      yarn build:icu-types
-      yarn build:compact-locales
-      yarn sass
-      yarn get-expire-time
-      yarn copy-components
-
-      # This is the equivalent of 'yarn build-linux' which also runs 'yarn generate'
+      npm run build-protobuf
+      npm run build:esbuild
+      npm run build:icu-types
+      npm run build:compact-locales
+      npm run sass
+      npm run get-expire-time
+      npm run copy-components
+
+      # This is the equivalent of 'npm run build-linux' which also runs 'npm run generate'
       # which is broken (as described above).
-      yarn build:esbuild:prod
-      yarn build:release --linux dir
+      npm run build:esbuild:prod
+      npm run build:release  -- --linux dir
 
       # Stage the built release. Directory is called 'linux-unpacked' for amd64,
       # and 'linux-arm64-unpacked' for arm64.