Use Makefile operator to get GOARCH default

* Fix bash syntax default for ENV var by using Makefile var syntax
* Provide a PR build time test to test the different architectures

.github/workflows/pr-collector.yml

@@ -0,0 +1,36 @@
+name: Build Collector on PR
+
+on:
+  pull_request:
+    branches: [ main ]
+
+jobs:
+  build-collector:
+    runs-on: ubuntu-20.04
+    name: Check if we can build the collector for ${{ matrix.architecture }}
+    strategy:
+      matrix:
+        architecture: [ amd64, arm64 ]
+    steps:
+      - name: Checkout this repo
+        uses: actions/checkout@v2
+      - name: Setup Go
+        uses: actions/setup-go@v2
+        with:
+          go-version: '^1.16'
+      - name: Build collector
+        run: GOARCH=${{ matrix.architecture }} make package
+        working-directory: collector
+      - name: Confirm architecture of built collector
+        working-directory: collector/build/extensions
+        run: |
+          if [ "${{ matrix.architecture }}" = "amd64" ]
+          then
+            SEARCH_STRING=x86
+          elif [ "${{ matrix.architecture }}" = "arm64" ]
+          then
+            SEARCH_STRING=ARM
+          else
+            exit 2
+          fi
+          grep "$SEARCH_STRING" <<< "$(file collector)"

collector/Makefile

@@ -5,7 +5,7 @@ LAYER_NAME:=otel-collector
 
 VERSION=$(shell cat VERSION)
 GIT_SHA=$(shell git rev-parse HEAD)
-GOARCH=${GOARCH-amd64}
+GOARCH ?= amd64
 GOBUILD=GO111MODULE=on CGO_ENABLED=0 installsuffix=cgo go build -trimpath
 BUILD_INFO_IMPORT_PATH=main