speed up tests by 20-30% by using GOGC=off

See the added comment for the rationale. For that same reason, I always
build Go itself via "GOGC=off ./make.bash", as it's noticeably faster.

Before this change:

	$ go clean -cache && go test -short
	PASS
	ok      mvdan.cc/garble 35.298s
	$ go test -short
	PASS
	ok      mvdan.cc/garble 2.703s

With the change:

	$ go clean -cache && go test -short
	PASS
	ok      mvdan.cc/garble 25.323s
	$ go test -short
	PASS
	ok      mvdan.cc/garble 2.469s

Incremental test runs with a warm cache are largely unaffected, as those
would run very few of those short-lived and allocation-heavy programs.

However, when the build cache isn't warm (such as when garble itself is
modified), we easily see savings of 20-30%.

We might revisit this in the future if Go's GC gets better in these
situations, which should make "go build" faster. For now, we run our
tests very often, so having them burn a bit less CPU is nice.

main_test.go

@@ -58,8 +58,25 @@ func TestScripts(t *testing.T) {
 		Dir: filepath.Join("testdata", "scripts"),
 		Setup: func(env *testscript.Env) error {
 			env.Vars = append(env.Vars,
+				// Use testdata/mod as our module proxy.
 				"GOPROXY="+proxyURL,
+
+				// We use our own proxy, so avoid sum.golang.org.
 				"GONOSUMDB=*",
+
+				// "go build" starts many short-lived Go processes,
+				// such as asm, buildid, compile, and link.
+				// They don't allocate huge amounts of memory,
+				// and they'll exit within seconds,
+				// so using the GC is basically a waste of CPU.
+				// Turn it off entirely, releasing memory on exit.
+				//
+				// We don't want this setting always on,
+				// as it could result in memory problems for users.
+				// But it helps for our test suite,
+				// as the packages are relatively small.
+				"GOGC=off",
+
 				"gofullversion="+runtime.Version(),
 			)