cmd/dist: "moved GOROOT" test failing on freebsd-arm builder

[paulzhol]

##### moved GOROOT
failed to initialize build cache at /home/paulzhol/.cache/go-build: mkdir /home/paulzhol/.cache: read-only file system

example log: https://build.golang.org/log/6efc1e3b5937210f65aa18552a990cd733145b3c

$HOME (and /) are readonly, GOCACHE is set (twice) in the buildlet environment:

:: Running /tmp/workdir-host-freebsd-arm-paulzhol/go/src/make.bash with args ["/tmp/workdir-host-freebsd-arm-paulzhol/go/src/make.bash"] and env ["BLOCKSIZE=K" "MAIL=/var/mail/paulzhol" "PATH=/sbin:/bin:/usr/sbin:/usr/bin:/usr/local/sbin:/usr/local/bin:/home/paulzhol/bin" "GO_TEST_TIMEOUT_SCALE=30" "CGO_ENABLED=1" "GOARM=7" "GOCACHE=/tmp/gocahe" "USER=paulzhol" "HOME=/home/paulzhol" "PWD=/" "RC_PID=21" "WORKDIR=/tmp/workdir-host-freebsd-arm-paulzhol" "GOROOT_BOOTSTRAP=/tmp/workdir-host-freebsd-arm-paulzhol/go1.4" "GO_BUILDER_NAME=freebsd-arm-paulzhol" "GOROOT_BOOTSTRAP=/usr/home/paulzhol/go1.4" "GOARM=7" "CGO_ENABLED=1" "GOBIN=" "TMPDIR=/tmp/workdir-host-freebsd-arm-paulzhol/tmp" "GOCACHE=/tmp/workdir-host-freebsd-arm-paulzhol/gocache"] in dir /tmp/workdir-host-freebsd-arm-paulzhol/go/src

[paulzhol]

The moved_goroot test explicitly filters GOCACHE from the environment:

go/src/cmd/dist/test.go

Lines 529 to 533 in 7cbfa55

	for _, e := range os.Environ() {
	if !strings.HasPrefix(e, "GOROOT=") && !strings.HasPrefix(e, "GOCACHE=") {
	cmd.Env = append(cmd.Env, e)
	}
	}

forcing the use of os.UserCacheDir() in cmd/go/internal/cache/default.go DefaultDir().

Switched to setting XDG_CACHE_HOME instead of GOCACHE for the buildlet environment.