Fix new-install symlink location

[fgaz]

Now new-install(-nonlocal-exes) should work without issues (if the ~/.cabal/config was generated with this)

Closes #5000 (yay!)

@23Skidoo can we cherry-pick it on 2.2 too?

Please include the following checklist in your PR:

• Patches conform to the coding conventions.
• Any changes that could be relevant to users have been recorded in the changelog.
  • the symlink-bindir default
• The documentation has been updated, if necessary.
  • https://cabal.readthedocs.io/en/latest/installing-packages.html#installation-paths
    • Not necessary since those are Cabal/Setup.hs paths
• If the change is docs-only, [ci skip] is used to avoid triggering the build bots.

Please also shortly describe how you tested your change. Bonus points for added tests!

[23Skidoo]

Sure, if it's new-build only, then it's low-risk.

[fgaz]

Yup. new-build and I set symlink-bindir (which is only used by new-install despite being there for literally 10 years) to ~/.cabal/bin by default in ~/.cabal/config

[hvr]

I don't give up hope on #680 ... maybe for 3.0? :-)

[fgaz]

Merging this now for the same reason as #4825
(travis was green. I stopped it because i forgot [ci skip] when force pushing the changelog)

[fgaz]

@23Skidoo fyi I just pushed it on 2.2 too