Fix build-ami workflow (#496)

* Improve build-ami trigger

* Add `packer init`

Author: 0x2b3bfa0

.github/workflows/build-ami.yml

@@ -1,13 +1,16 @@
 name: build-ami
 on: 
   push:
-    tags: ['ami*']
+    branches: [master]
+    paths: [environment/**]
 jobs:
   build-ami:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
-      - run: packer build environment
+      - run: |
+          packer init environment
+          packer build environment
         env:
             AWS_SECRET_ACCESS_KEY: ${{ secrets.AWS_SECRET_ACCESS_KEY }}
             AWS_ACCESS_KEY_ID: ${{ secrets.AWS_ACCESS_KEY_ID }}