ci: copy python.exe to python3.exe on Windows

Some of our CI scripts use `#!/usr/bin/env python3` which works fine on
Linux and macOS, but fails on Windows as only python.exe (pointing to
Python 3) is present. To avoid the failure this commit adds a step that
copies python.exe to python3.exe.

Author: pietroalbini

src/ci/azure-pipelines/steps/run.yml

@@ -42,6 +42,10 @@ steps:
       fi
     displayName: Decide whether to run this job
 
+- bash: cp $(which python) $(dirname $(which python))/python3.exe
+  displayName: Copy python.exe to python3.exe
+  condition: and(succeeded(), eq(variables['Agent.OS'], 'Windows_NT'))
+
 # Spawn a background process to collect CPU usage statistics which we'll upload
 # at the end of the build. See the comments in the script here for more
 # information.