Compiler detection: avoid false recognition of MSVC (#34574)

Interim fix for #34559

Spack's MSVC compiler definition uses ifx as the Fortran compiler.
Prior to #33385, the Spack MSVC compiler definition required the
executable to be called "ifx.exe"; #33385 replaced this with just
"ifx", which inadvertently led to ifx falsely indicating the
presence of MSVC on non-Windows systems (which leads to future
errors when attempting to query/use those compiler objects).

This commit applies a short-term fix by updating MSVC Fortran
version detection to always indicate a failure on non-Windows.

lib/spack/spack/compilers/msvc.py

@@ -160,6 +160,8 @@ def setup_custom_environment(self, pkg, env):
     def fc_version(cls, fc):
         # We're using intel for the Fortran compilers, which exist if
         # ONEAPI_ROOT is a meaningful variable
+        if not sys.platform == "win32":
+            return "unknown"
         fc_ver = cls.default_version(fc)
         avail_fc_version.add(fc_ver)
         fc_path[fc_ver] = fc