rustfmt incompatible with `unused-qualifications`

[palfrey]

Our repository has -Dunused-qualifications in the build flags, and now we can't run rustfmt anymore (well, without some workarounds) because we get lots of errors like the following when running bazel run @rules_rust//:rustfmt:

error: unnecessary qualification
   --> external/rules_rust+/rust/runfiles/runfiles.rs:158:49
    |
158 |         let mode = if let Some(manifest_file) = std::env::var_os(MANIFEST_FILE_ENV_VAR) {
    |                                                 ^^^^^^^^^^^^^^^^
    |
    = note: requested on the command line with `-D unused-qualifications`
help: remove the unnecessary path segments
    |
158 -         let mode = if let Some(manifest_file) = std::env::var_os(MANIFEST_FILE_ENV_VAR) {
158 +         let mode = if let Some(manifest_file) = env::var_os(MANIFEST_FILE_ENV_VAR) {
    |