buildAmici.sh: Only try installing numpy if not yet available

[dweindl]

Makes buildAmici.sh work with system-python. Before, it would fail if we don't have write permissions to system-packages even if numpy was already installed.

[codecov]

Codecov Report

Merging #2230 (427c2fd) into develop (ecbae3f) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #2230   +/-   ##
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  Coverage    76.64%   76.64%           
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  Files           91       91           
  Lines        15109    15109           
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  Hits         11580    11580           
  Misses        3529     3529           

Flag	Coverage Δ
cpp	73.11% <ø> (ø)
cpp_python	37.08% <ø> (ø)
petab	53.74% <ø> (ø)
python	77.44% <ø> (ø)
sbmlsuite	?

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[FFroehlich]

👍 check against some minimum version?

[dweindl]

check against some minimum version?

Wouldn't hurt, but it's a bit of a pain. We'll either have to hardcode minimum numpy versions for different python versions and platforms here, or we need to parse the constraints from pyproject.toml.
It would be easiest if pip would support installing only build-dependencies, but that's not possible unfortunately.

I would rather address that some other time. This PR doesn't really change anything in that respect. Before this change, numpy wouldn't have been upgraded either. With system-pip, pip install numpy would already fail before even checking if there is any numpy installed.