ci: fix pip branch install on forks

Before this patch, pip branch install pipelines had failed if CI is
triggered with a PR from a fork [1].

1. https://github.com/tarantool/tarantool-python/actions/runs/4052831981/jobs/7162061474

Author: DifferentialOrange

Diff for: .github/workflows/testing.yml

@@ -212,7 +212,7 @@ jobs:
         run: python3 .github/scripts/remove_source_code.py
 
       - name: Install the package with pip
-        run: pip3 install git+$GITHUB_SERVER_URL/$GITHUB_REPOSITORY@$GITHUB_REF_NAME
+        run: pip3 install git+$GITHUB_SERVER_URL/$GITHUB_REPOSITORY@$GITHUB_REF
 
       - name: Install test requirements
         run: pip3 install -r requirements-test.txt
@@ -340,7 +340,7 @@ jobs:
         run: python3 .github/scripts/remove_source_code.py
 
       - name: Install the package with pip
-        run: pip3 install git+$env:GITHUB_SERVER_URL/$env:GITHUB_REPOSITORY@$env:GITHUB_REF_NAME
+        run: pip3 install git+$env:GITHUB_SERVER_URL/$env:GITHUB_REPOSITORY@$env:GITHUB_REF
 
       - name: Install test requirements
         run: pip3 install -r requirements-test.txt