buildcache create `--allow-root` is default in current spack versions

[simonpintarelli]

As pointed out by @albestro. from spack changelog (https://github.com/spack/spack/blob/develop/CHANGELOG.md#L313)

buildcache push: make `--allow-root` the default and deprecate the option (#38878)

It was announced to remove --allow-root in spack v0.22, although it is still there with the warning:

    if args.allow_root:
        tty.warn(
            "The flag `--allow-root` is the default in Spack 0.21, will be removed in Spack 0.22"
        )