Values without a backing env var should not be expanded (#368)

* Values without a backing env var should not be expanded

* Add unit tests for sourcing behavior

* Replace godotenv with shell lib

.github/workflows/unit.yml

@@ -0,0 +1,21 @@
+name: Run Unit Tests
+
+on:
+  push:
+    branches:
+      - main
+  pull_request:
+
+jobs:
+  build:
+    runs-on: ubuntu-22.04
+    steps:
+      - uses: actions/checkout@v4
+      - name: Setup Go
+        uses: actions/setup-go@v4
+        with:
+          go-version: '1.22.x'
+      - name: Install dependencies
+        run: go mod download
+      - name: Test with the Go CLI
+        run: go test -v ./...