feat(core): switch to script bootstrap as a default

[aignas]

This PR enables the bootstrap script implementation for bazel 8 and above
for UNIX platforms.

DO NOT MERGE: this has to be discussed:

• Version number bump to 2.0?

Work towards #2156
Work towards #2521
Relands #2858 and reverts #2968
Fixes#2983

Related:

• Long term solution in PEP 796: Relative Virtual Environments python/peps#4476.
• Support for rules_pkg to produce working tars is in Support packaging raw symlinks from declare_symlink bazelbuild/rules_pkg#929 that has been released in 1.2.
• The support for working tars that has symlinks is not yet ready in tar.bzl without a heavy performance penalty, see [FR]: tar rule should support runfiles with symlinks tar.bzl#16
• rules_img appears to support symlinks: Symlinks in runfiles are not handled rules_img#254

[aignas]

FYI, @rickeylev, @alexeagle , we can continue talking about re-enabling the script=bootstrap by default for bazel 8+ here. Is there a good way to detect that we are running on that particular version with something else than config.none attribute check?

[rickeylev]

Wait for PEP 796

PEPs can take a long time, so I don't think we should wait on that.

Wait for rules_pkg 1.2 release

Maybe. If the 1.2 release is coming soon, then waiting sounds OK. If it takes too long, though, then I think we'll just have to press ahead. At the least, we should wait until there exists some rule that can package them.

[shayanhoshyari]

This is a bit of a mind puzzle 😅 . Might as well wait for 7 to drop and just do the simple windows -> system_python, posix -> script

[aignas]

Yeah, I think with bazel 9 released we will do this.