chore: checkout corresponding branch instead of commit (#476)

Author: josecorella

Diff for: codebuild/release/prod-release.yml

@@ -1,8 +1,6 @@
 version: 0.2
 
 env:
-  variables:
-    BRANCH: "master"
   secrets-manager:
     TWINE_USERNAME: PyPiAdmin:username 
     TWINE_PASSWORD: PyPiAdmin:password
@@ -16,6 +14,7 @@ phases:
       python: latest
   pre_build:
     commands:
+      - git fetch origin $COMMIT_ID:$COMMIT_ID --depth=1
       - git checkout $COMMIT_ID
       - FOUND_VERSION=$(sed -n 's/__version__ = "\(.*\)"/\1/p' src/aws_encryption_sdk/identifiers.py)
       - |

Diff for: codebuild/release/test-release.yml

@@ -1,8 +1,6 @@
 version: 0.2
 
 env:
-  variables:
-    BRANCH: "master"
   secrets-manager:
     TWINE_USERNAME: TestPyPiCryptoTools:username 
     TWINE_PASSWORD: TestPyPiCryptoTools:password
@@ -16,6 +14,7 @@ phases:
       python: latest
   pre_build:
     commands:
+      - git fetch origin $COMMIT_ID:$COMMIT_ID --depth=1
       - git checkout $COMMIT_ID
       - FOUND_VERSION=$(sed -n 's/__version__ = "\(.*\)"/\1/p' src/aws_encryption_sdk/identifiers.py)
       - |