minor tweaks in preparation for Go 1.21

Update CI to use a newer version of Go master,
now that we're already getting release candidates.

Look at the diffs between Go 1.20 and master of `go help build`
and `go help testflag`, and add two flags that were recently added.

While here, bump a hopeful TODO for a feature request,
since that one definitely did not happen for 1.21.

.github/workflows/test.yml

@@ -91,7 +91,7 @@ jobs:
     - uses: actions/checkout@v3
     - name: Install Go
       env:
-        GO_COMMIT: 363713223385476b87dc5f26d267df8c67d13006 # 2023-06-03
+        GO_COMMIT: a031f4ef83edc132d5f49382bfef491161de2476 # 2023-06-24
       run: |
         cd $HOME
         mkdir $HOME/gotip

main.go

@@ -356,7 +356,7 @@ func mainErr(args []string) error {
 
 		// Until https://github.com/golang/go/issues/50603 is implemented,
 		// manually construct something like a pseudo-version.
-		// TODO: remove when this code is dead, hopefully in Go 1.21.
+		// TODO: remove when this code is dead, hopefully in Go 1.22.
 		if mod.Version == "(devel)" {
 			var vcsTime time.Time
 			var vcsRevision string
@@ -2107,6 +2107,7 @@ var forwardBuildFlags = map[string]bool{
 	"-buildmode":     true,
 	"-compiler":      true,
 	"-cover":         true,
+	"-covermode":     true,
 	"-coverpkg":      true,
 	"-gccgoflags":    true,
 	"-gcflags":       true,
@@ -2149,6 +2150,7 @@ var booleanFlags = map[string]bool{
 	"-benchmem": true,
 	"-c":        true,
 	"-failfast": true,
+	"-fullpath": true,
 	"-json":     true,
 	"-short":    true,
 }