cmd/dist: delete unnecessary dirs from GOROOT on completion

On my machine, these directories add up to 276mb
and account for 40% of the size of the GOROOT directory.

Once bootstrapping is complete, they are never used again.

Fixes #31851

Change-Id: Idbf8f21bae3d64655aa43761cc778677add6234a
Reviewed-on: https://go-review.googlesource.com/c/go/+/175377
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/dist/build.go

@@ -496,6 +496,7 @@ func setup() {
 		xremoveall(p)
 	}
 	xmkdirall(p)
+	xatexit(func() { xremoveall(p) })
 
 	// Create tool directory.
 	// We keep it in pkg/, just like the object directory above.

src/cmd/dist/buildtool.go

@@ -128,6 +128,7 @@ func bootstrapBuildTools() {
 	// but it is easier to debug on failure if the files are in a known location.
 	workspace := pathf("%s/pkg/bootstrap", goroot)
 	xremoveall(workspace)
+	xatexit(func() { xremoveall(workspace) })
 	base := pathf("%s/src/bootstrap", workspace)
 	xmkdirall(base)