Add parse error location to unparsable environment file warning

[wasimat404]

Fixes #11963.

When a GHC environment file cannot be parsed, cabal previously warned only that the file "is unparsable", giving no indication of where or why. The underlying parser already produces a Parsec error carrying the line, column, and the unexpected/expected tokens, but that information was discarded at the warning site in getExistingEnvEntries. This surfaces it.

Before:

Warning: The environment file /home/user/.ghc/.../environments/default is unparsable. Libraries cannot be installed.

After:

Warning: The environment file /home/user/.ghc/.../environments/default is unparsable. Libraries cannot be installed.
"/home/user/.ghc/.../environments/default" (line 1, column 1):
unexpected 't'
expecting "--", "package-id", "global-package-db", "user-package-db", "package-db", "clear-package-db" or end of input

The reporter's case was a stray trailing newline, which the parser reports as "unexpected end of input" at the offending line, so this change points directly at it. The fix is message-only; parser behaviour is unchanged.

Added a cabal-testsuite package test under PackageTests/EnvironmentFile/UnparsableWarning that points cabal at a malformed environment file and asserts the warning includes the location. Verified the test fails on unpatched cabal (the location line is absent) and passes with the change.

QA Notes

Create a malformed GHC environment file, e.g. a file bad.env whose first line is this is not valid. Then run:

cabal install --lib base --package-env=bad.env

The warning should name the parse location, for example:

Warning: The environment file bad.env is unparsable. Libraries cannot be installed.
"bad.env" (line 1, column 1):
unexpected 't'
expecting "--", "package-id", ...

Before this change, only the first line ("is unparsable") was printed, with no location.

[wasimat404]

@zlonast Thanks for the nudge:)