Ensure that the PEX_ROOT is always a realpath.

[benjyw]

In almost all codepaths, a pex_root will be a realpath. See, e.g.,

pex/pex/pex_info.py

Line 511 in 06b8850

pex_root = os.path.realpath(os.path.expanduser(self.raw_pex_root))

pex/pex/variables.py

Line 320 in 06b8850

return os.path.realpath(os.path.expanduser(value))

However there was one codepath by which a non-realpath could
be obtained, namely boot() callingVariables.PEX_ROOT.value_or()
with a raw_pex_root given as the fallback value.

This change ensures that Variables.PEX_ROOT is always a realpath.

This manifested as a bug at pex boot time in the presence of a
symlinked cache dir: pantsbuild/pants#21321

[benjyw]

I've verified that this fixes pantsbuild/pants#21321

[jsirois]

@benjyw thanks for figuring this one out and fixing. I'm going to merge and get a release prepped.