cmd/go: do not use cache when -a is specified

Clearly -a means don't use the cache.
An oversight that it did.

Fixes #22586.

Change-Id: I250b351439bd3fb5f8d6efc235b614f0a75ca64c
Reviewed-on: https://go-review.googlesource.com/76016
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>

src/cmd/go/go_test.go

@@ -4728,6 +4728,9 @@ func TestBuildCache(t *testing.T) {
 	tg.run("build", "-x", "complex/w")
 	tg.grepStderrNot(`[\\/]compile|gccgo`, "ran compiler incorrectly")
 
+	tg.run("build", "-a", "-x", "complex/w")
+	tg.grepStderr(`[\\/]compile|gccgo`, "did not run compiler with -a")
+
 	// complex is a non-trivial main package.
 	// the link step should not be cached.
 	tg.run("build", "-o", os.DevNull, "-x", "complex")

src/cmd/go/internal/work/buildid.go

@@ -355,17 +355,19 @@ func (b *Builder) useCache(a *Action, p *load.Package, actionHash cache.ActionID
 	// We treat hits in this cache as being "stale" for the purposes of go list
 	// (in effect, "stale" means whether p.Target is up-to-date),
 	// but we're still happy to use results from the build artifact cache.
-	if c := cache.Default(); c != nil {
-		outputID, size, err := c.Get(actionHash)
-		if err == nil {
-			file := c.OutputFile(outputID)
-			info, err1 := os.Stat(file)
-			buildID, err2 := buildid.ReadFile(file)
-			if err1 == nil && err2 == nil && info.Size() == size {
-				a.built = file
-				a.Target = "DO NOT USE - using cache"
-				a.buildID = buildID
-				return true
+	if !cfg.BuildA {
+		if c := cache.Default(); c != nil {
+			outputID, size, err := c.Get(actionHash)
+			if err == nil {
+				file := c.OutputFile(outputID)
+				info, err1 := os.Stat(file)
+				buildID, err2 := buildid.ReadFile(file)
+				if err1 == nil && err2 == nil && info.Size() == size {
+					a.built = file
+					a.Target = "DO NOT USE - using cache"
+					a.buildID = buildID
+					return true
+				}
 			}
 		}
 	}