go: avoid host contamination by GOCACHE

By default GOCACHE is set to $HOME/.cache.

Fixes:
ERROR: go-cross-dbfp4-1.12.1-r0 do_compile: Function failed: do_compile (log file is located at /workdir/build/tmp/work/x86_64-linux/go-cross-dbfp4/1.12.1-r0/temp/log.do_compile.8120)
ERROR: Logfile of failure stored in: /workdir/build/tmp/work/x86_64-linux/go-cross-dbfp4/1.12.1-r0/temp/log.do_compile.8120
Log data follows:
| DEBUG: Executing shell function do_compile
| Building Go cmd/dist using /workdir/build/tmp/work/x86_64-linux/go-cross-dbfp4/1.12.1-r0/recipe-sysroot-native/usr/lib/go.
| failed to initialize build cache at /home/pokyuser/.cache/go-build: mkdir /home/pokyuser/.cache: permission denied
| WARNING: exit code 1 from a shell command.
| ERROR: Function failed: do_compile (log file is located at /workdir/build/tmp/work/x86_64-linux/go-cross-dbfp4/1.12.1-r0/temp/log.do_compile.8120)
ERROR: Task (/workdir/repo/poky/meta/recipes-devtools/go/go-cross_1.12.bb:do_compile) failed with exit code '1'
NOTE: Tasks Summary: Attempted 23 tasks of which 16 didn't need to be rerun and 1 failed.

Signed-off-by: Ricardo Ribalda Delgado <ricardo@ribalda.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>