ci: MSVC fails after rebuild of Docker image

[real-or-random]

As part of testing #1320, I forced a rebuild of the Docker image (after the successful CI run on the old image, i.e., after @hebasto's ACK.) I think we should anyway do this from time to time...

Now MSVC refuses to run because it can't write some temp files. See for example:
https://github.com/bitcoin-core/secp256k1/pull/1320/checks?check_run_id=13686337961

Some resources:

• https://www.winehq.org/pipermail/wine-users/2007-June/027383.html
• https://stackoverflow.com/questions/10560779/cl-exe-when-launched-via-createprocess-does-not-seem-to-have-write-permissions