Use fvm in devenv again. (#1726)

Needed for #1703 (or upgrading flutter in general right now), because
using the current nix version of flutter 3.24 lead me to this bug:
flutter/flutter#153238 and I don't want to
block upgrading flutter further, since until the nix version of flutter
is upgraded it will probably take a week or more.

devenv.nix

@@ -9,17 +9,6 @@
 { pkgs, inputs, lib, ... }:
 
 let
-  pkgs-unstable = import inputs.nixpkgs-unstable { system = pkgs.stdenv.system; };
-  flutter = pkgs-unstable.flutter.overrideAttrs
-    {
-      src = pkgs-unstable.fetchFromGitHub
-        {
-          owner = "flutter";
-          repo = "flutter";
-          rev = "3.22.2";
-          sha256 = "7ndnIw72YxNB+VeeejEeRD+xxuLXOcWo322s5CMWzBM=";
-        };
-    };
   androidenv = android-pkgs.androidenv.override {
     # Seems to only work when androidenv.buildApp is used 
     # afterwards (we don't).
@@ -56,7 +45,6 @@ in
   env = {
     ANDROID_HOME = "${android-sdk-root}";
     ANDROID_SDK_ROOT = "${android-sdk-root}";
-    FLUTTER_ROOT = "${flutter}";
     CHROME_EXECUTABLE = "${pkgs.google-chrome}/bin/google-chrome-stable";
   };
 
@@ -66,7 +54,9 @@ in
     pkgs.nixpkgs-fmt
 
     android-sdk
-    flutter
+
+    # Running `fvm flutter` -> 'Missing "unzip" tool. Unable to extract Dart SDK.'
+    pkgs.unzip
 
     # For Flutter Web development
     pkgs.google-chrome
@@ -80,11 +70,23 @@ in
   enterShell = ''
     # Make pub cache work so that we can execute
     # e.g. `dart pub global activate fvm`
-    #export PATH="$PATH":"$HOME/.pub-cache/bin"
+    export PATH="$PATH":"$HOME/.pub-cache/bin"
 
     # Make sz cli work
     export PATH="$PATH":"$DEVENV_ROOT/bin"
 
+    if ! command -v fvm &> /dev/null
+    then
+        echo "fvm could not be found. Installing FVM via dart pub global."
+        dart pub global activate fvm
+        # fvm install will fail getting dependencies if we dont
+        # cd to app. Using && will only change it for the fvm
+        # install command
+        cd app && fvm install
+    fi
+
+    fvm flutter config --android-sdk ${android-sdk-root}
+
     # When running e.g. `sz pub get -c 0` the analytics
     # will cause the task to fail (maybe because so many signals
     # are sent in a short time). So we just disable analytics
@@ -96,7 +98,7 @@ in
     # start running e.g. `sz pub get` right away. Without this 
     # one would first have to run pub get in the sz cli folder 
     # manually.
-    dart pub get --directory ./tools/sz_repo_cli
+    fvm dart pub get --directory ./tools/sz_repo_cli
   '';
 
   languages = {
@@ -105,6 +107,9 @@ in
       enable = true;
       jdk.package = pkgs.jdk17;
     };
+
+    # Use this so that we can run "dart pub global activate fvm"
+    dart.enable = true;
   };
 
   # See full reference at https://devenv.sh/reference/options/