Ensure non-ambiguous args/env vars injection into PEXes (Cherry-pick of #18861)

[huonw]

This fixes #18779 by ensuring that injecting arguments and environment variables into a pex doesn't risk them being interpreted by the pex CLI itself if they start with -, and ensuring they're quoted appropriately to match the shlex.split within pex (https://github.com/pantsbuild/pex/blob/ff220b9e41484450e0c78136f98d6899f2e2325c/pex/bin/pex.py#L433).

The issue noted in #18779 was specifically ambiguity with args, but the same problem will apply to env and so that is fixed here too (although it's rather less common to have env vars starting with -). NB. shlex.quoteing alone won't handle the - ambiguity, shlex.quote("--foo") == "--foo".

For a target like pex_binary(..., args=["--foo"], env={"--bar": "baz"}):

• Previously, this would invoke pex ... --inject-args --foo --inject-env --bar=baz, and argparse within the pex CLI will intepret --foo and --bar=... as independent parameters, rather than arguments to --inject-args/--inject-env.
• After this change (as suggested by @jsirois), the invocation will be pex ... --inject-args=--foo --inject-env=--bar=baz and there's no risk of misinterpretation.

For a target like pex_binary(..., args=["spaces 'n' quotes"]):

• Previously this would invoke with (equivalently) an arg --inject-args=spaces 'n' quotes, which would shlex.split to three arguments in the PEX, the same as passing args=["spaces", "n", "quotes"])
• Now, this will invoke with --inject-args='spaces '"'"'n'"'"' quotes', which, is correctly shlex.split to the single value: spaces 'n' quotes.