tool: Switch back to mtime

[dasJ]

We kinda need this because XFS doesn't appear to support birth time:

running 15 tests
test esp::tests::nixos_path_creates_correct_filename_from_nix_store_path ... ok
test gc::tests::delete_empty_unused_directory ... ok
test gc::tests::delete_unused_directory_with_unused_file_inside ... ok
test gc::tests::delete_unused_file ... ok
test generation::tests::parse_version_correctly ... ok
test gc::tests::keep_used_file ... ok
test os_release::tests::escaping_works ... ok
test gc::tests::only_delete_filtered_unused_files ... ok
test os_release::tests::parses_correctly_from_str ... ok
test gc::tests::keep_used_directory_with_used_and_unused_file ... ok
test pe::tests::convert_to_valid_uefi_path ... ok
test pe::tests::convert_to_valid_uefi_path_relative_to_esp ... ok
test systemd::tests::compare_version_correctly ... ok
test systemd::tests::fail_to_parse_version ... ok
test systemd::tests::parse_version_correctly ... ok

test result: ok. 15 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

     Running tests/gc.rs (target/release/deps/gc-0baf904aefee7e61)

running 2 tests
test keep_only_configured_number_of_generations ... ok
test keep_unrelated_files_on_esp ... ok

test result: ok. 2 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.37s

     Running tests/install.rs (target/release/deps/install-5a1b29efda2ec3fe)

running 3 tests
test overwrite_unsigned_files ... ok
test do_not_install_duplicates ... ok
test overwrite_unsigned_images ... ok

test result: ok. 3 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.07s

     Running tests/os_release.rs (target/release/deps/os_release-e408e464bb6ad5d2)

running 1 test
test generate_expected_os_release ... FAILED

failures:

---- generate_expected_os_release stdout ----
Error: creation time is not available for the filesystem


failures:
    generate_expected_os_release

test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

cc @nikstur @blitz

[RaitoBezarius]

LGTM