JS shell: invalid flags

[hsyl20]

In the JS shell:

nix develop github:input-output-hk/devx#ghc810-js-minimal-iog --no-write-lock-file --refresh

we have:

env | grep NIX_CABAL_FLAGS
NIX_CABAL_FLAGS=--with-ghc=javascript-unknown-ghcjs-ghc --with-ghc-pkg=javascript-unknown-ghcjs-ghc-pkg --with-hsc2hs=javascript-unknown-ghcjs-hsc2hs

Two issues:

• ghcjs is built with prefix "js-unknown-ghcjs", not "javascript-unknown-ghcjs" (while the latter is correct for the JS backend)
• "--ghcjs" isn't passed to cabal (only required for ghcjs, not for the JS backend)

[hsyl20]

Cc @JoshMeredith

[hsyl20]

We should probably test the environment on CI if possible.