Add missing pip in $PATH on Cygwin CI

This makes a pip -> pip3 symlink in /usr/bin in a new step prior to
the first step that runs the pip command. Using "pip3", "pip3.9",
or a command like "python -m pip" would work, but this allows the
Cygwin workflow to continue using the same installation commands as
the main testing workflow.

Adding this fixes two problems:

1. When the pip version installed by the python39-pip package is
   current, so that upgrading pip doesn't install any new commmand,
   no "pip" command is created in /usr/local. This has happened not
   to be the case for a long time, which is why the Cygwin workflow
   was able to pass. (That the recent failures started at the merge
   of gitpython-developers#1783 turns out to be a coincidence: rerunning jobs on prior
   commits has the failure, as does experimentally reverting it.)

2. Even when the pip version installed by python39-pip is behind
   the latest available version, pip is still used before being
   upgraded to check if setuptools is installed, to decide whether
   to upgrade it. This is to keep similar steps in the two testing
   workflows similar, since the Cygwin workflow only uses Python
   3.9, which always has setuptools. Because pip was never in $PATH
   in that step, the Cygwin workflow wrongly refrained from trying
   to upgrade setuptools.

When the "Update PyPA packages" step does find a newer version of
pip to upgrade to, it installs it in /usr/local/bin, which we have
in $PATH before /usr/bin, so the upgraded version, when present,
will still be preferred in subsequent commands, as before.

Running "pip" on Cygwin when it may not be in $PATH -- and, for one
step, never is -- was a bug introduced in e8956e5 (gitpython-developers#1709). Before
that, "pip" still was not always available, but it was not used.
This change fixes the bug by making sure "pip" is always available.

.github/workflows/cygwin-test.yml

@@ -55,6 +55,11 @@ jobs:
         # and cause subsequent tests to fail
         cat test/fixtures/.gitconfig >> ~/.gitconfig
 
+    - name: Ensure the "pip" command is available
+      run: |
+        # This is used unless, and before, and updated pip is installed.
+        ln -s pip3 /usr/bin/pip
+
     - name: Update PyPA packages
       run: |
         # Get the latest pip, wheel, and prior to Python 3.12, setuptools.