to make it easier to figure out why it fails.
For example in https://github.com/systemd/systemd/runs/4799774735?check_suite_focus=true
it failed with
```
meson.build:1003:8: ERROR: Command "/usr/bin/clang -print-targets" failed with status 1.
A full log can be found at /home/runner/work/systemd/systemd/build/meson-logs/meson-log.txt
Error: Process completed with exit code 1.
```
and it wasn't clear what exactly happened there.
Let's assign a specific -Dcryptolib= value to each job to have at least
some coverage for all supported cryptolibs without unnecessarily
multiplying the test matrix.
Should provide coverage for #21880.
ATTOW llvm-11 got into focal-updates, which conflicts with llvm-11
provided by the apt.llvm.org repositories. Let's use the system
llvm package if available in such cases to avoid that.
It seems there is another meson (0.57.0) regression preventing clang from
building systemd with --optimization=3 -Db_lto=true
By analogy with https://github.com/google/oss-fuzz/pull/5199 let's just switch
to 0.56.2 for the time being