ci-cygwin: Fix rebasing

[mkoeppe]

https://github.com/sagemath/sage/runs/7491424384?check_suite_focus=true

D:                14G  2.3G   12G  16% /cygdrive/d
Getting list of dlls. This may take a while...
Now rebasing...
usage: rebaseall
Error: Process completed with exit code 1.

Depends on #34216
Depends on #34213

Component: porting: Cygwin

Author: Matthias Koeppe

Branch/Commit: u/mkoeppe/ci_cygwin__fix_rebasing @ 7240b3a

Reviewer: https://github.com/mkoeppe/sage/actions/runs/2736055965

Issue created by migration from https://trac.sagemath.org/ticket/34223

[mkoeppe]

Dependencies: #34216

[mkoeppe]

Changed dependencies from #34216 to #34216, #34213

[mkoeppe]

Branch: u/mkoeppe/ci_cygwin__fix_rebasing

[mkoeppe]

New commits:

942639d	build/pkgs/{gfortran,openssl,python3}: On Cygwin, do not try to build the SPKG
e99c3f6	Merge #34216
fb2c9d2	build/pkgs/sagelib/spkg-install: Only unset SAGE_LOCAL and SAGE_PKG_CONFIG_PATH, do not poison them
5d11ff7	build/pkgs/sagelib/spkg-install [CYGWIN]: Rebase in SAGE_VENV, not SAGE_LOCAL
dedb533	Merge #34213
7240b3a	.github/workflows/cygwin.yml: Pass SAGE_LOCAL to extract-sage-local.sh

[mkoeppe]

Commit: 7240b3a

[mkoeppe]

Author: Matthias Koeppe

[mkoeppe]

Reviewer: https://github.com/mkoeppe/sage/actions/runs/2736055965