compiletest: only take a `--python` argument and remove the `--lldb-python` / `--docck-python` args

[jyn514]

Compiletest currently has two different arguments telling it which python to use. Since #95441, both are always the same. We should change it to just one --python argument to simplify things.

Mentoring instructions: Change

rust/src/tools/compiletest/src/main.rs

Lines 64 to 65 in f38c5c8

	.reqopt("", "lldb-python", "path to python to use for doc tests", "PATH")
	.reqopt("", "docck-python", "path to python to use for doc tests", "PATH")

to be a single python argument, then change

rust/src/bootstrap/test.rs

Lines 1403 to 1405 in f38c5c8

	cmd.arg("--docck-python").arg(builder.python());
	cmd.arg("--lldb-python").arg(builder.python());

to use the updated arguments.

@AlecGoncharow are you interested in tackling this one too? :)

@rustbot label +A-rustbuild +A-testsuite +E-easy +E-mentor

Originally posted by @jyn514 in #95441 (comment)

[AlecGoncharow]

@rustbot claim