cmd/go/internal/work: omit modinfo line from cache key when empty

Cache keys are dumped in case of mismatch; an empty modinfo string
adds noise to that dump without a corresponding benefit.

For #52009.

Change-Id: I1b4cd85fa5ff920973552fc94977954f93622a32
Reviewed-on: https://go-review.googlesource.com/c/go/+/398059
Trust: Bryan Mills <[email protected]>
Run-TryBot: Bryan Mills <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>

Author: Bryan C. Mills

src/cmd/go/internal/work/exec.go

@@ -303,7 +303,9 @@ func (b *Builder) buildActionID(a *Action) cache.ActionID {
 			fmt.Fprintf(h, "fuzz %q\n", fuzzFlags)
 		}
 	}
-	fmt.Fprintf(h, "modinfo %q\n", p.Internal.BuildInfo)
+	if p.Internal.BuildInfo != "" {
+		fmt.Fprintf(h, "modinfo %q\n", p.Internal.BuildInfo)
+	}
 
 	// Configuration specific to compiler toolchain.
 	switch cfg.BuildToolchainName {