CI: cygwin coverage uploader needs to run in bash too

Like every single other command in the entire workflow, and for the same
reasons. ;)

.github/workflows/cygwin.yml

diff --git a/.github/workflows/cygwin.yml b/.github/workflows/cygwin.yml
index 5b1cd95..68c99c7 100644
--- a/.github/workflows/cygwin.yml
+++ b/.github/workflows/cygwin.yml
@@ -105,3 +105,4 @@
 
       - name: Upload coverage report
         run: ./ci/upload_cov.sh "${{ matrix.NAME }}"
+        shell: C:\cygwin\bin\bash.exe --noprofile --norc -o igncr -eo pipefail '{0}'