fix CI builds

[AntonioBL]

This should solve the issue of nightly builds not properly generated for Windows.
"grep -P" was causing an error in Appveyor build chain (I checked by connecting via Remote Desktop to one of Appveyor builds).
I added the same modifications also for Mac to test if the absence of Mac nightly builds is due to the same cause.

• I signed CLA
• I made sure the code in the PR follows the coding rules
• [only Win checked] I made sure the code compiles on my machine
• [Mac to be tested] I made sure there are no unnecessary changes in the code
• I made sure the title of the PR reflects the core meaning of the issue you are solving
• I made sure the commit message(s) contain a description and answer the question "Why do those changes fix that particular issue?" or "Why are those changes really necessary as improvements?"
• [N/A] I made sure the commit message title starts with "fix #424242:" if there is a related issue
• [N/A] I created the test (mtest, vtest, script test) to verify the changes I made

[Jojo-Schmitz]

I believe just for consistency those changes from grep -oP "VERSION\s+\K.*" to sed -n -e "s/^.*VERSION *//p" should be done to Makefile line 23 too.

[AntonioBL]

Well, apparently under Linux there are no errors, so there is no need to change. The result of the commands should basically be the same.
If it ain't broke, don't fix it. 😉

[AntonioBL]

By the way, for Windows this PR works, see:
https://ci.appveyor.com/project/MuseScore/musescore/builds/32127503#L3420
(and following lines)
Under Mac we can't verify since Mac building of PRs is disabled.

[Jojo-Schmitz]

Well, apparently under Linux there are no errors, so there is no need to change. The result of the commands should basically be the same.
If it ain't broke, don't fix it.

As said, (just) for consistency. If another change is needed, all relevant places would be easier to spot.

By the way, for Windows this PR works, see:
https://ci.appveyor.com/project/MuseScore/musescore/builds/32127503#L3420
(and following lines)
Under Mac we can't verify since Mac building of PRs is disabled.

Still I'm pretty sure this fixes the Mac issue too, looking at the output of a 'regular' (i.e. non-PR) build, which complains about that grep -P, twice. See for example https://travis-ci.org/github/musescore/MuseScore/jobs/674069975#L6187-L6191 and https://travis-ci.org/github/musescore/MuseScore/jobs/674069975#L6429-L6432