build.sh: Add support for build flags

Currently, it is not possible to adjust the build process for personal
needs.

With this patch it will be possible to add compiler and linker flags as
well as adjust the name of the output if the CMD_PATH != PROJECT_NAME,
e.g. "cmd/foo/main.go".

build.sh

@@ -10,6 +10,7 @@ ln -s $GITHUB_WORKSPACE $PROJECT_ROOT
 cd $PROJECT_ROOT
 go get -v ./...
 
+BUILD_FLAGS=''
 EXT=''
 
 if [ $GOOS == 'windows' ]; then
@@ -19,7 +20,7 @@ fi
 if [ -x "./build.sh" ]; then
   OUTPUT=`./build.sh "${CMD_PATH}"`
 else
-  go build "${CMD_PATH}"
+  go build ${BUILD_FLAGS} "${CMD_PATH}"
   OUTPUT="${PROJECT_NAME}${EXT}"
 fi