tests/test-cli-extensions: Fix with single-binary coreutils

tests/test-cli-extensions: Fix with single-binary coreutils

On systems where `coreutils` are built with `--enable-single-binary=symlinks` like Nix,
`/usr/bin/env` is symlinked to `/usr/bin/coreutils` and uses `argv[0]` to determine which program to run.
Since the `test-cli-extensions.sh` created a new symlink named `ostree-env`,
coreutils would be confused about the utility to choose, so running it would fail:

	ostree-env: unknown program ‘ostree-env’
	Try 'ostree-env --help' for more information.

Fixes: https://github.com/ostreedev/ostree/issues/2553