run the mhs version probe without the package path flag

[dmjio]

tldr;

mcabal -a<dir> install fails for any package with a library because the version probe in mhsNameVers becomes mhs -a'<dir>' --numeric-version, which mhs rejects; this change executes a plain mhs --numeric-version, the same form mhsBackend already uses at startup.

This change will require a follow-up PR to regen MicroCabal in MicroHs.

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mhsNameVers asks the compiler for its version by running mhs --numeric-version. That command was built with mhsOut, the helper used for compiler invocations, which prepends -a'PATH' when mcabal was started with -a (a package database other than ~/.mcabal). The probe then became

mhs -a'PATH' --numeric-version

and mhs only accepts --numeric-version as its sole argument, so it fails with

mhs: uncaught exception: error: Unknown flag: --numeric-version

mhsNameVers is reached through getMhsDir, which is used when installing a library, so 'mcabal -aPATH install' failed for every package with a library. Using -a is needed e.g. when mhs is installed read-only by a package manager (nix) and packages have to go into a writable database.

The package path has no bearing on the compiler version, so run the probe as a plain mhs --numeric-version (through cmdOut with the compiler executable), the same way mhsBackend already does at startup.