Set GOPATH in build script for Go module cache

Set GOPATH to ${outdir} so that Go module cache gets put there.
diff --git a/cli/README.md b/cli/README.md
index b6843c0..db3b143 100644
--- a/cli/README.md
+++ b/cli/README.md
@@ -61,7 +61,7 @@
   - ```mvn -Dtarget=cross -Dos=OS -Darch=ARCH clean install```  build for platform with operating system OS and architecture ARCH
 
 This builds the requested binaries into the `target/` directory, each in its own subdirectory with a name that includes 
-the platform/architecture details, e.g. `bin/linux.386/br`.  The build installs a maven artifact to the maven repository,
+the platform/architecture details, e.g. `bin/linux.386/br`. (When using this build process, the build script also writes the Go module cache into this directory.) The build installs a maven artifact to the maven repository,
 consisting of a zip file containing all the binaries.  This artifact can be referenced in a POM as
 
 ```xml
diff --git a/cli/release/build.sh b/cli/release/build.sh
index aec675d..e6cd91f 100755
--- a/cli/release/build.sh
+++ b/cli/release/build.sh
@@ -140,8 +140,8 @@
 
 echo "Starting build.sh (brooklyn-client go build script)"
 
-# Just in case user has accidentally defined GOPATH to include this directory
-unset GOPATH
+# Set GOPATH to ${outdir} so that Go module cache gets put there.
+export GOPATH=${outdir}
 
 #
 # Test if go is available