bzltestutil: fix re-exec failure on Windows when runfiles path exceeds MAX_PATH (#4586)

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**What type of PR is this?**

Bug fix

**What does this PR do? Why is it needed?**

Fix `go_test` failures when the path containing the test executable is
longer than `MAX_PATH`.
The issue stems from `CreateProcess` which will systematically reject
any working directory (whether passed explicitly or to be inherited)
longer than `MAX_PATH`, regardless of longpath prefix (`\\?\`), long
path setting being enabled in the registry, or `longPathAware` being
present in the binary's manifest.

**Which issues(s) does this PR fix?**

AFAIK, there is no open issue.

I initially reported this to go's tracker as
https://github.com/golang/go/issues/78601, but in the context of
`rules_go` I don't think there is one.

**Other notes for review**

I wasn't sure what to do about testing. AFAICS testing `Wrap` might
quickly become over complicated, and testing the new condition in
isolation doesn't bring much. Please let me know if you have something
in mind here!

---------

Co-authored-by: Joseph Gette <jgettepost@gmail.com>
Co-authored-by: Fabian Meumertzheim <fabian@meumertzhe.im>