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.

(From OE-Core rev: 9a6d208b9979035bbfc1def80fb6558db4bddb12)

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

meta/classes/go.bbclass

@@ -7,6 +7,7 @@ GOROOT_class-nativesdk = "${STAGING_DIR_TARGET}${libdir}/go"
 GOROOT = "${STAGING_LIBDIR}/go"
 export GOROOT
 export GOROOT_FINAL = "${libdir}/go"
+export GOCACHE = "${B}/.cache"
 
 export GOARCH = "${TARGET_GOARCH}"
 export GOOS = "${TARGET_GOOS}"

meta/recipes-devtools/go/go-cross.inc

@@ -14,6 +14,7 @@ export GO386 = "${TARGET_GO386}"
 export GOMIPS = "${TARGET_GOMIPS}"
 export GOROOT_BOOTSTRAP = "${STAGING_LIBDIR_NATIVE}/go"
 export GOROOT_FINAL = "${libdir}/go"
+export GOCACHE = "${B}/.cache"
 CC = "${@d.getVar('BUILD_CC').strip()}"
 
 do_configure[noexec] = "1"

meta/recipes-devtools/go/go-runtime.inc

@@ -15,6 +15,7 @@ export CGO_CFLAGS = "${CFLAGS}"
 export CGO_CPPFLAGS = "${CPPFLAGS}"
 export CGO_CXXFLAGS = "${CXXFLAGS}"
 export CGO_LDFLAGS = "${LDFLAGS}"
+export GOCACHE = "${B}/.cache"
 
 GO_EXTLDFLAGS ?= "${HOST_CC_ARCH}${TOOLCHAIN_OPTIONS} ${LDFLAGS}"
 GO_SHLIB_LDFLAGS ?= '-ldflags="--linkmode=external -extldflags '${GO_EXTLDFLAGS}'"'

meta/recipes-devtools/go/go-target.inc

@@ -10,6 +10,7 @@ export GO386 = "${TARGET_GO386}"
 export GOMIPS = "${TARGET_GOMIPS}"
 export GOROOT_BOOTSTRAP = "${STAGING_LIBDIR_NATIVE}/go"
 export GOROOT_FINAL = "${libdir}/go"
+export GOCACHE = "${B}/.cache"
 GO_LDFLAGS = ""
 GO_LDFLAGS_class-nativesdk = "-linkmode external"
 export GO_LDFLAGS