[test-release] fallback to py3's venv module

If virtualenv is not available, we can look for py3's venv instead. We only
use this particular env for installing and running the test suite.

utils/release/test-release.sh

@@ -502,12 +502,18 @@ fi
 # Setup the test-suite.  Do this early so we can catch failures before
 # we do the full 3 stage build.
 if [ $do_test_suite = "yes" ]; then
+  venv=virtualenv
+  if ! type -P 'virtualenv' > /dev/null 2>&1 ; then
+    check_program_exists 'python3'
+    venv="python3 -m venv"
+  fi
+
   SandboxDir="$BuildDir/sandbox"
   Lit=$SandboxDir/bin/lit
   TestSuiteBuildDir="$BuildDir/test-suite-build"
   TestSuiteSrcDir="$BuildDir/llvm-test-suite"
 
-  virtualenv $SandboxDir
+  ${venv} $SandboxDir
   $SandboxDir/bin/python $BuildDir/llvm-project/llvm/utils/lit/setup.py install
   mkdir -p $TestSuiteBuildDir
 fi