Merge #17068: qt: Always generate bitcoinstrings.cpp on make translate

Summary:
c640ca24f9d94f468ec6de023bf885813ac6c9a9 Always generate bitcoinstrings.cpp on make translate (Franck Royer)

Pull request description:

Resolves #16891.

`bitcoinstrings.cpp` is to be generated at release time. Hence,
it should not depend on whether the source files are younger as the
releaser may proceed from a fresh checkout.

More information on the investigation in the issue.

ACKs for top commit:

jonasschnelli:
  Tested ACK c640ca24f9d94f468ec6de023bf885813ac6c9a9 - Tested master + this on macOS 10.14. With this PR, it always "runs" `GEN      qt/bitcoinstrings.cpp` and `GEN      translate`.

Tree-SHA512: f799fdc3ad16a2a6a59704bc2c50f5179e6a7e064d8f43354592f11857cc901cac99b2b90f3319d25d49c9d78378b8d119cc5f59b48ea7f1008f33dd26700877

Backport of Core PR17068

According to the PR discussion, this issue may affect make translate on MacOS and Ubuntu.

Reviewer note: This is out of order, as PR16291 should ideally come first. But I don't want to tackle that backport
at this time.

Depends on D5790 for the test plan.

Test Plan:

../configure
make -j8
cd src
make translate
arc lint

Not committed with this patch, but there should be some sane output in bitcoinstrings.cpp

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5791