Flaky clippy test: undocumented_unsafe_blocks_fixable

This came up in #148478, build log:

    FAILED TEST: tests/ui-toml/undocumented_unsafe_blocks/undocumented_unsafe_blocks_fixable.disabled.fixed
command: CLIPPY_CONF_DIR="tests/ui-toml/undocumented_unsafe_blocks/disabled" RUSTC_ICE="0" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2-tools/x86_64-unknown-linux-gnu/release/clippy-driver" "--error-format=json" "--emit=metadata" "-Aunused" "-Ainternal_features" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "-Dwarnings" "-Ldependency=/checkout/obj/build/x86_64-unknown-linux-gnu/stage2-tools/release/deps" "--sysroot=/checkout/obj/build/x86_64-unknown-linux-gnu/stage2" "--out-dir" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2-tools/ui_test/0/tests/ui-toml/undocumented_unsafe_blocks" "tests/ui-toml/undocumented_unsafe_blocks/undocumented_unsafe_blocks_fixable.disabled.fixed" "--extern" "proc_macro_unsafe=/checkout/obj/build/x86_64-unknown-linux-gnu/stage2-tools/ui_test/0/tests/ui/auxiliary/libproc_macro_unsafe.so" "-L" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2-tools/ui_test/0/tests/ui/auxiliary" "--extern" "futures=/checkout/obj/build/x86_64-unknown-linux-gnu/stage2-tools/x86_64-unknown-linux-gnu/debug/deps/libfutures-a8bf6f69c9b80e37.rlib" "--extern" "futures=/checkout/obj/build/x86_64-unknown-linux-gnu/stage2-tools/x86_64-unknown-linux-gnu/debug/deps/libfutures-a8bf6f69c9b80e37.rmeta" "--extern" "itertools=/checkout/obj/build/x86_64-unknown-linux-gnu/stage2-tools/x86_64-unknown-linux-gnu/debug/deps/libitertools-2d82d3ce68d9c3ee.rlib" "--extern" "itertools=/checkout/obj/build/x86_64-unknown-linux-gnu/stage2-tools/x86_64-unknown-linux-gnu/debug/deps/libitertools-2d82d3ce68d9c3ee.rmeta" "--extern" "libc=/checkout/obj/build/x86_64-unknown-linux-gnu/stage2-tools/x86_64-unknown-linux-gnu/debug/deps/liblibc-e86cd1a7c22fc60c.rlib" "--extern" "libc=/checkout/obj/build/x86_64-unknown-linux-gnu/stage2-tools/x86_64-unknown-linux-gnu/debug/deps/liblibc-e86cd1a7c22fc60c.rmeta" "--extern" "parking_lot=/checkout/obj/build/x86_64-unknown-linux-gnu/stage2-tools/x86_64-unknown-linux-gnu/debug/deps/libparking_lot-a94abfa769f04e5d.rlib" "--extern" "parking_lot=/checkout/obj/build/x86_64-unknown-linux-gnu/stage2-tools/x86_64-unknown-linux-gnu/debug/deps/libparking_lot-a94abfa769f04e5d.rmeta" "--extern" "quote=/checkout/obj/build/x86_64-unknown-linux-gnu/stage2-tools/x86_64-unknown-linux-gnu/debug/deps/libquote-22b65b7753b00e23.rlib" "--extern" "quote=/checkout/obj/build/x86_64-unknown-linux-gnu/stage2-tools/x86_64-unknown-linux-gnu/debug/deps/libquote-22b65b7753b00e23.rmeta" "--extern" "regex=/checkout/obj/build/x86_64-unknown-linux-gnu/stage2-tools/x86_64-unknown-linux-gnu/debug/deps/libregex-4368476ea091f8ef.rlib" "--extern" "regex=/checkout/obj/build/x86_64-unknown-linux-gnu/stage2-tools/x86_64-unknown-linux-gnu/debug/deps/libregex-4368476ea091f8ef.rmeta" "--extern" "serde=/checkout/obj/build/x86_64-unknown-linux-gnu/stage2-tools/x86_64-unknown-linux-gnu/debug/deps/libserde-978a92b8a77cdf3d.rlib" "--extern" "serde=/checkout/obj/build/x86_64-unknown-linux-gnu/stage2-tools/x86_64-unknown-linux-gnu/debug/deps/libserde-978a92b8a77cdf3d.rmeta" "--extern" "syn=/checkout/obj/build/x86_64-unknown-linux-gnu/stage2-tools/x86_64-unknown-linux-gnu/debug/deps/libsyn-fdddf18d670fbd00.rlib" "--extern" "syn=/checkout/obj/build/x86_64-unknown-linux-gnu/stage2-tools/x86_64-unknown-linux-gnu/debug/deps/libsyn-fdddf18d670fbd00.rmeta" "--extern" "tokio=/checkout/obj/build/x86_64-unknown-linux-gnu/stage2-tools/x86_64-unknown-linux-gnu/debug/deps/libtokio-1fc6a815de90452d.rlib" "--extern" "tokio=/checkout/obj/build/x86_64-unknown-linux-gnu/stage2-tools/x86_64-unknown-linux-gnu/debug/deps/libtokio-1fc6a815de90452d.rmeta" "-L" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2-tools/debug/deps" "-L" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2-tools/x86_64-unknown-linux-gnu/debug/deps" "--edition" "2024" "--crate-name" "__undocumented_unsafe_blocks_fixable_1"
error: test got exit status: 1, but expected 0
 = note: after rustfix is applied, all errors should be gone, but weren't
full stderr:
error: failed to create encoded metadata from file: No such file or directory (os error 2)
error: aborting due to 1 previous error
full stdout:
FAILURES:
    tests/ui-toml/undocumented_unsafe_blocks/undocumented_unsafe_blocks_fixable.disabled.fixed

Cc @flip1995 @oli-obk (I wonder if this is a race in the ui_test rustfix integration)

Start with tests/ui-toml/undocumented_unsafe_blocks/undocumented_unsafe_blocks_fixable.disabled.fixed and the linked build log, then inspect the ui_test rustfix integration implicated by the report. Reproduce the metadata-creation failure and determine why the expected output is unavailable. Done means the test is reliable and passes repeatedly, including after rustfix is applied.