[buildgen] Ensure virtualenv is installed (#35624)

[Manual run](https://fusion2.corp.google.com/ci/kokoro/prod:grpc%2Fcore%2Fmaster%2Flinux%2Fgrpc_build_boringssl_at_head/activity/d33676a9-7bf8-452f-a720-77cca2950c22/log)

Closes #35624

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/35624 from gnossen:fix_fix_protobuf e8613764d2
PiperOrigin-RevId: 600601009

requirements.bazel.txt

@@ -23,9 +23,9 @@ proto-plus==1.22.3
 google-auth==2.23.4
 googleapis-common-protos==1.61.0
 cachetools==5.3.2
-charset-normalizer==3.3.2                                                                         
+charset-normalizer==3.3.2
 pyasn1==0.5.0
-rsa==4.9                                                   
+rsa==4.9
 greenlet==1.1.3.post0
 zope.interface==6.1
 opentelemetry-sdk==1.21.0

tools/buildgen/generate_projects.sh

@@ -36,7 +36,7 @@ tools/buildgen/build_cleaner.py build_handwritten.yaml
 
 # /usr/local/google/home/rbellevi/dev/tmp/grpc/venv/bin/python3: No module named virtualenv
 # Generate xds-protos
-[[ -d generate_projects_virtual_environment ]] || python3 -m virtualenv generate_projects_virtual_environment
+[[ -d generate_projects_virtual_environment ]] || { python3 -m pip install virtualenv --upgrade && python3 -m virtualenv generate_projects_virtual_environment; }
 generate_projects_virtual_environment/bin/pip install --upgrade --ignore-installed grpcio-tools==1.59.0
 generate_projects_virtual_environment/bin/python tools/distrib/python/xds_protos/build.py