TUN-8546: rework MacOS build script

The rework consists in building and packaging the cloudflared binary based on the OS & ARCH of the system.

read TARGET_ARCH from export and exit if TARGET_ARCH is not set

.teamcity/mac/build.sh

@@ -7,6 +7,12 @@ if [[ "$(uname)" != "Darwin" ]] ; then
     exit 1
 fi
 
+if [[ "amd64" != "${TARGET_ARCH}" && "arm64" != "${TARGET_ARCH}" ]]
+then
+  echo "TARGET_ARCH must be amd64 or arm64"
+  exit 1
+fi
+
 go version
 export GO111MODULE=on
 
@@ -137,13 +143,10 @@ fi
 
 # cleanup the build directory because the previous execution might have failed without cleaning up.
 rm -rf "${TARGET_DIRECTORY}"
-archs=("amd64" "arm64")
-export TARGET_OS=darwin
-for arch in ${archs[@]}; do
-  
-  FILENAME="$(pwd)/artifacts/cloudflared-darwin-$arch.tgz"
-  PKGNAME="$(pwd)/artifacts/cloudflared-$arch.pkg"
-  TARGET_ARCH=$arch GOCACHE="$PWD/../../../../" GOPATH="$PWD/../../../../" CGO_ENABLED=1 make cloudflared
+export TARGET_OS="darwin"
+FILENAME="$(pwd)/artifacts/cloudflared-darwin-$TARGET_ARCH.tgz"
+PKGNAME="$(pwd)/artifacts/cloudflared-$TARGET_ARCH.pkg"
+GOCACHE="$PWD/../../../../" GOPATH="$PWD/../../../../" CGO_ENABLED=1 make cloudflared
 
 # sign the cloudflared binary
 if [[ ! -z "$CODE_SIGN_NAME" ]]; then
@@ -186,7 +189,6 @@ for arch in ${archs[@]}; do
       --install-location /usr/local/bin \
       ${PKGNAME}
 fi
-done
 
 # cleanup build directory because this script is not ran within containers,
 # which might lead to future issues in subsequent runs.