sage/doctest/forker.py: don't say "warning" when running under CI

The Github CI annotates each "Warning, slow doctest" as an error.
This is distracting, and there's no way to disable it. So when running
under the CI, we now print "Uh oh" instead of "Warning". This will
allow the slow doctests to remain detected, but won't clutter up
the Github UI with what look like errors but aren't.

Author: orlitzky

src/sage/doctest/forker.py

@@ -1516,7 +1516,10 @@ def report_overtime(self, out, test, example, got, *, check_timer=None):
 
         OUTPUT: prints a report to ``out``
 
-        EXAMPLES::
+        EXAMPLES:
+
+        Run an example with the Github CI variables overridden to test
+        the "normal" warning output::
 
             sage: from sage.doctest.parsing import SageOutputChecker
             sage: from sage.doctest.forker import SageDocTestRunner
@@ -1534,15 +1537,49 @@ def report_overtime(self, out, test, example, got, *, check_timer=None):
             sage: check = Timer()
             sage: check.cputime = 2.34
             sage: check.walltime = 3.12
+            sage: from os import environ
+            sage: environ["GITHUB_ACTIONS"] = ""
+            sage: environ["CI"] = ""
             sage: DTR.report_overtime(sys.stdout.write, doctests[0], ex, 'BAD ANSWER\n', check_timer=check)
             **********************************************************************
             File ".../sage/doctest/forker.py", line 12, in sage.doctest.forker
             Warning, slow doctest:
                 doctest_var = 42; doctest_var^2
             Test ran for 1.23s cpu, 2.50s wall
             Check ran for 2.34s cpu, 3.12s wall
+
+        Repeat the example above as if we're running under the Github CI::
+
+            sage: from os import environ
+            sage: environ["GITHUB_ACTIONS"] = "true"
+            sage: environ["CI"] = "true"
+            sage: DTR.report_overtime(sys.stdout.write, doctests[0], ex, 'BAD ANSWER\n', check_timer=check)
+            **********************************************************************
+            File ".../sage/doctest/forker.py", line 12, in sage.doctest.forker
+            Uh oh, slow doctest:
+                doctest_var = 42; doctest_var^2
+            Test ran for 1.23s cpu, 2.50s wall
+            Check ran for 2.34s cpu, 3.12s wall
         """
-        out(self._failure_header(test, example, 'Warning, slow doctest:') +
+        # Github actions (1) annotate "Warning" as an error, and (2)
+        # provide no way to disable this for files that you didn't
+        # actually change:
+        #
+        #   https://github.com/actions/toolkit/issues/559
+        #
+        # As a very silly workaround, we avoid the word "Warning" for
+        # long-running tests when running under the CI. (We don't want
+        # to display a litany of "Error" notations that have nothing
+        # to do with the present pull request.)
+        #
+        # In the test below, keep in mind that both bool(None) and bool("")
+        # evaluate to False.
+        warning_word = "Warning"
+        from os import environ
+        if environ.get('CI') or environ.get('GITHUB_ACTIONS'):
+            warning_word = "Uh oh"
+
+        out(self._failure_header(test, example, f'{warning_word}, slow doctest:') +
             ('Test ran for %.2fs cpu, %.2fs wall\nCheck ran for %.2fs cpu, %.2fs wall\n'
              % (example.cputime,
                 example.walltime,