fix cmake warning when building with -DMYSQL=ON

[BlaiZephyr]

closes #7881

edit:
the ubuntu 20.04 check is failing - i cant exactly tell why, i assume its because of:

CMake Error at CMakeLists.txt:642 (message):
You must install MySQL to compile the DDNet server with MySQL support

Checklist

• Tested the change ingame
• Provided screenshots if it is a visual change
• Tested in combination with possibly related configuration options
• Written a unit test (especially base/) or added coverage to integration test
• Considered possible null pointers and out of bounds array indexing
• Changed no physics that affect existing maps
• Tested the change with ASan+UBSan or valgrind's memcheck (optional)

[heinrich5991]

I don't get why the check fails. But it doesn't fail on master so I assume it's got something to do with this PR.

[Robyt3]

Permalink for run results: logs_58358.zip

[rffontenelle]

In ubuntu-20.04 I noticed this output:

CMake Warning:
  Value of MYSQL_CONFIG_INCLUDE_DIR contained a newline; truncating

I also noticed the version difference:

• In ubuntu-latest: cmake 3.22.1-1ubuntu1.22.04.1
• In ubuntu-20.04: cmake 3.16.3-1ubuntu1.20.04.1

[BlaiZephyr]

status: no idea how to approach this - feel free to close if you dont want an open PR lying around

[Jupeyy]

when ubuntu 24 will be added to the CIs we can also drop ubuntu 20.

see actions/runner-images#9691

[rffontenelle]

ubuntu-24.04 now available, although it is not ubuntu-latest yet

actions/runner-images#9848