Omit -u option when building on podman.

[pilhuhn]

Passing the -u option to podman makes make docker-generate fail if OCI_BIN is set to podman.
Thanks for @mmat11 providing me a link to the Cilium build script
Closes: #1311

[mariomac]

Good job! Thank you very much for your contribution

[mmat11]

lgtm!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 44.30%. Comparing base (3047a0a) to head (28bc62c).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1312      +/-   ##
==========================================
+ Coverage   43.53%   44.30%   +0.76%     
==========================================
  Files         305      304       -1     
  Lines       32864    27743    -5121     
==========================================
- Hits        14307    12291    -2016     
+ Misses      17638    14718    -2920     
+ Partials      919      734     -185     

Flag	Coverage Δ
integration-test	?
integration-test-arm	?
k8s-integration-test	?
oats-test	?
unittests	44.30% <ø> (-0.01%)	⬇️

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