Fix the cache key for v5 release and master environment

[BoxiLi]

Description
The conda environment cache key used qutip-${{ matrix.qutip-version }}, which evaluated to the same string qutip-5 for both the v5-release and v5-master jobs. Because the conda cache stores the entire environment directory, including pip-installed packages, whichever job saved its cache last would set the qutip version for both jobs on subsequent runs. As a result, qutip-v5-release does not get its own environment but uses the qutip-v5-master env instead.

Changed the cache key suffix from qutip-${{ matrix.qutip-version }} to ${{ matrix.label }}, giving each job (v4-release, v5-release, v5-master) its own independent conda environment cache.

[hodgestar]

Oops. This probably explains a bunch of strange flakiness in the test runs. :/

[BoxiLi]

There is something that I still don't understand.

According to the qutip.about(), some notebooks are using the release version, some the dev version

https://nbviewer.org/urls/qutip.org/qutip-tutorials/tutorials-v5/quantum-circuits/qip-toffoli-cnot.ipynb
At this moment uses qutip 5.3.0.dev0+c9f293f while

https://nbviewer.org/urls/qutip.org/qutip-tutorials/tutorials-v5/quantum-circuits/teleportation.ipynb
uses qutip 5.2.3