CI: Install git on Cygwin CI runner

Cygwin pip now has a chance to resolve everything on the command line.  

It won't be able to resolve dependencies, due to something pulling in Rust, but it'll get to the point where pip points out that it is not pip's fault that CI doesn't have Rust compilers for Cygwin

.github/workflows/main.yml

@@ -58,6 +58,7 @@ jobs:
             gcc-core,
             gcc-g++,
             ncompress
+            git
       - name: Run tests
         shell: C:\cygwin\bin\env.exe CYGWIN_NOWINPATH=1 CHERE_INVOKING=1 C:\cygwin\bin\bash.exe -leo pipefail -o igncr {0}
         run: tox