Add `UV_NO_BUILD_ISOLATION` as environment variable (#3318)
## Summary
Hi! Added `UV_NO_BUILD_ISOLATION` as a boolean environment variable for
the `--no-build-isolation` command-line option.
Closes https://github.com/astral-sh/uv/issues/3309
## Test Plan
Added new test `respect_no_build_isolation_env_var` to check that the
behaviour is the same as if the ``--no-build-isolation``
command-line-option is set.
