[Fix #11850] Build dependencies of build tool on disk too

[Saizan]

This PR fixes #11850 by taking the simple approach of ensuring the build tool can be properly built.

Checklist:

• Patches conform to the coding conventions.
• Any changes that could be relevant to users have been recorded in the changelog.
  • Is the change significant? If so, remember to add significance: significant in the changelog file.
• The documentation has been updated, if necessary.
• Manual QA notes have been included.
• Tests have been added. (Ask for help if you don’t know how to write them! Ask for an exemption if tests are too complex for too little coverage!)

[Mikolaj]

Thank you for the PR. Please mention the issue it fixes in the PR descriptions and please mark the PR with review_needed label once ready.

[Mikolaj]

Oh, and please add the changelog snippet.

[Saizan]

@Mikolaj should be all there now, let me know if the changelog needs editing.

[Mikolaj]

The changelog is perfect, thank you.

What can go wrong? Does this impair performance in some scenarios? Can the location that is used for the build be read-only? Anything else?

[Saizan]

• The build of the tool can still fail for unrelated reasons (which is why it would be useful to have something like a "keep-going" switch that HLS could use to still have a session to load even if some things will fail/use a stale tool)
• We do build some dependencies on disk which we also load at the repl, so extra time is spent doing that, but I don't see a way around it. Will not impact resource usage of the repl itself though.

Can the location that is used for the build be read-only?

Like the subdirectory for the unit under $builddir ? I suppose so once the build is done, but will need writing once cabal realizes it's stale and needs building again. Does Cabal make such dirs read-only?

• Another concern will be to keep it so the repl doesn't pick up on the built on-disk version of the library, otherwise the repl won't be the same.