Blacklist the GOMACC_PATH environment variable

The clang compiler wrapper will be looking for this environment
variable, but it should only ever be set by the `gomacc` in CC_WRAPPER.
Make sure that it isn't otherwise set in the environment.

Change-Id: I4b26c7183bc47f71dffa7dfcc2e5b08e840a11b1

core/goma.mk

@@ -14,6 +14,9 @@
 # limitations under the License.
 #
 
+# Used by the compiler wrapper, but should only be set by gomacc
+unexport GOMACC_PATH
+
 # Notice: this works only with Google's Goma build infrastructure.
 ifneq ($(filter-out false,$(USE_GOMA)),)
   # Goma requires a lot of processes and file descriptors.