Upload test logs with a UTF-8 content type

[fmeum]

This fixes rendering of test .log files when opened in a browser.

Updating the deps was required to get the project to compile due to:

error[E0635]: unknown feature `proc_macro_span_shrink`
  --> /Users/fmeum/.cargo/registry/src/index.crates.io-6f17d22bba15001f/proc-macro2-1.0.56/src/lib.rs:92:30
   |
92 |     feature(proc_macro_span, proc_macro_span_shrink)
   |                              ^^^^^^^^^^^^^^^^^^^^^^

Update command:

cargo update proc-macro2 --precise 1.0.60

[fmeum]

Does this require a release/image update of some kind?

[coeuvre]

Got error with this change:

Nov 27 15:10:10.119 ERROR bazelci_agent::artifact::upload: Failed to execute command `buildkite-agent artifact upload src/test/java/com/google/devtools/build/lib/skyframe/SequencedSkyframeExecutorTest/test.log --content_type=text/plain;encoding=utf-8`: exit status exit status: 1

The correct usage is:

buildkite-agent artifact upload [options] <pattern> [destination]