Changes

gen_makefile: python: run build in venv too

The format of the license field in pyproject.toml was changed from a
"table" to a a string with an SPDX license expression. The pyosmocom
sources have been adjusted to use the new format to appease a warning
("By 2026-Feb-18, you need to update your project and remove deprecated
calls or your builds will no longer be supported."). However now the
build fails with older setuptools versions.

Run the build of python packages in the venv too, to ensure we have a
recent setuptools version.

Related: pyosmocom I8416234ea02f337fabd075cc345af964c1b71adf
Related: osmo-ci Ia8ef3a0301aeaad4691c197ed780e86b497def4a
Related: https://packaging.python.org/en/latest/guides/writing-pyproject-toml/#license-and-license-files
Change-Id: Idbd826d65267c05dcf2d82f7429d53a52686d737