Upload test logs with a UTF-8 content type

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
```

agent/src/artifact/upload.rs

@@ -210,7 +210,7 @@ fn upload_bep_json_file(
     build_event_json_file: &Path,
     mode: Mode,
 ) -> Result<()> {
-    uploader.upload_artifact(dry, None, build_event_json_file, mode)
+    uploader.upload_artifact(dry, None, build_event_json_file, mode, Some("application/json"))
 }
 
 fn gen_error_content(bazelci_task: &str, label: &str, name: &str, test_log: &str) -> String {
@@ -329,7 +329,7 @@ fn parse_test_xml(path: &Path, bazelci_task: &str, label: &str) -> Result<Option
     Ok(Some(writer.into_inner().into_inner()))
 }
 
-fn execute_command(dry: bool, cwd: Option<&Path>, program: &str, args: &[&str]) -> Result<()> {
+fn execute_command(dry: bool, cwd: Option<&Path>, program: &str, args: &Vec<&str>) -> Result<()> {
     println!("{} {}", program, args.join(" "));
 
     if dry {
@@ -384,6 +384,7 @@ impl Uploader {
         cwd: Option<&Path>,
         artifact: &Path,
         mode: Mode,
+        content_type: Option<&str>,
     ) -> Result<()> {
         {
             let file = match cwd {
@@ -405,7 +406,7 @@ impl Uploader {
         }
 
         match mode {
-            Mode::Buildkite => self.upload_artifact_buildkite(dry, cwd, artifact),
+            Mode::Buildkite => self.upload_artifact_buildkite(dry, cwd, artifact, content_type),
         }
     }
 
@@ -414,13 +415,19 @@ impl Uploader {
         dry: bool,
         cwd: Option<&Path>,
         artifact: &Path,
+        content_type: Option<&str>,
     ) -> Result<()> {
         let artifact = artifact.display().to_string();
+        let mut args = vec!["artifact", "upload", artifact.as_str()];
+        let ct_arg = content_type.map(|ct| "--content_type=".to_owned() + ct).unwrap_or_default();
+        if !ct_arg.is_empty() {
+            args.push(ct_arg.as_str());
+        }
         execute_command(
             dry,
             cwd,
             "buildkite-agent",
-            &["artifact", "upload", artifact.as_str()],
+            &args,
         )
     }
 
@@ -631,7 +638,7 @@ fn upload_test_log(
     mode: Mode,
 ) -> Result<()> {
     let (cwd, artifact) = resolve_artifact(test_log, local_exec_root)?;
-    return uploader.upload_artifact(dry, cwd.as_ref().map(|pb| pb.as_path()), &artifact, mode);
+    return uploader.upload_artifact(dry, cwd.as_ref().map(|pb| pb.as_path()), &artifact, mode, Some("text/plain;encoding=utf-8"));
 }
 
 fn upload_test_xml(

agent/tests/artifact/upload.rs

@@ -19,7 +19,7 @@ fn test_logs_uploaded_to_buildkite() -> Result<()> {
     ]);
     cmd.assert()
         .success()
-        .stdout(predicates::str::contains("buildkite-agent artifact upload src\\test\\shell\\bazel\\resource_compiler_toolchain_test\\test.log"));
+        .stdout(predicates::ord::eq("buildkite-agent artifact upload src\\test\\shell\\bazel\\resource_compiler_toolchain_test\\test.log --content_type=text/plain;encoding=utf-8\n"));
 
     Ok(())
 }
@@ -37,7 +37,7 @@ fn test_logs_uploaded_to_buildkite() -> Result<()> {
     ]);
     cmd.assert()
         .success()
-        .stdout(predicates::str::contains("buildkite-agent artifact upload src/test/shell/bazel/starlark_repository_test/shard_4_of_6/test_attempts/attempt_1.log"));
+        .stdout(predicates::ord::eq("buildkite-agent artifact upload src/test/shell/bazel/starlark_repository_test/shard_4_of_6/test_attempts/attempt_1.log --content_type=text/plain;encoding=utf-8\n"));
 
     Ok(())
 }