cmd/go: set CCACHE_DIR when setting HOME to non-existent dir

cmd/go: set CCACHE_DIR when setting HOME to non-existent dir

This fixes systems for which ccache is the default compiler.

Also remove a couple of temporary files created by TestImportMain.

Fixes #17668.

Change-Id: I1edefdcec5f417be0533c146253c35ff4928c1c0
Reviewed-on: https://go-review.googlesource.com/32328
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>