For Github Action CI, pass action's Python 3 in test call

The prior patch worked in my local environment, but missed a nuance of
Python 3.8 being available in Github's runner, though 3.6 is the
action's configured version.  The Makefile PYTHON3 variable does a
newest-version-first search, so virtualenv wasn't found at runtime.

The name of the action's Python 3 command was found via the home page of
the setup-python action:
https://github.com/marketplace/actions/setup-python

Signed-off-by: Alex Nelson <alexander.nelson@nist.gov>

Author: ajnelson-nist

.github/workflows/ci.yml

@@ -38,4 +38,4 @@ jobs:
     - name: Start from clean state
       run: make clean
     - name: Run tests
-      run: make check
+      run: make PYTHON3=python check

Makefile

@@ -13,6 +13,8 @@
 
 SHELL := /bin/bash
 
+PYTHON3 ?= $(shell which python3.9 2>/dev/null || which python3.8 2>/dev/null || which python3.7 2>/dev/null || which python3.6 2>/dev/null || which python3)
+
 all:
 
 .PHONY: \
@@ -38,6 +40,7 @@ all:
 check: \
   .git_submodule_init.done.log
 	$(MAKE) \
+	  PYTHON3=$(PYTHON3) \
 	  --directory tests \
 	  check