SCons: Bump min version to 3.1.2, test it on CI with one Linux job

The min SCons version had to be bumped as SCons 3.0 before 3.0.3 seems
broken (see #92043), and there's little gain from supporting 3.0.3-3.0.5.

3.1.2 is also the first version to avoid ambiguities between Python 2
and Python 3 usage, so we finally use it as the minimum baseline.

Also test against Python 3.6 which is also our minimum supported version.
This should help prevent regressions whenever we modernize the build scripts.

SConstruct

@@ -1,6 +1,6 @@
 #!/usr/bin/env python
 
-EnsureSConsVersion(3, 0, 0)
+EnsureSConsVersion(3, 1, 2)
 EnsurePythonVersion(3, 6)
 
 # System