CI: make cuda tests even run to begin with

Arch profile.d scripts were converted to use an appending function that
disappears when /etc/profile exits, and overall are simply not suitable
-- any more -- for sourcing individually.

(I will freely admit I'm not really sure what the overall goal of
refraining from sourcing /etc/profile itself is. Arguably it's kind of
misuse of the profile...)

This silently broke the cuda tests, which never ran because the cuda
compiler was not detected as available.

While we are at it, I guess we can convert gentoo to use the same trick
of appending it in install.sh

ci/ciimage/build.py

@@ -80,11 +80,6 @@ def gen_bashrc(self) -> None:
             fi
         '''
 
-        if self.data_dir.name == 'gentoo':
-            out_data += '''
-                source /etc/profile
-            '''
-
         out_file.write_text(out_data, encoding='utf-8')
 
         # make it executable

ci/ciimage/cuda/install.sh

@@ -18,4 +18,4 @@ install_minimal_python_packages
 # Manually remove cache to avoid GitHub space restrictions
 rm -rf /var/cache/pacman
 
-echo "source /etc/profile.d/cuda.sh" >> /ci/env_vars.sh
+echo "source /etc/profile" >> /ci/env_vars.sh

ci/ciimage/gentoo/install.sh

@@ -156,3 +156,5 @@ rm /usr/lib/python/EXTERNALLY-MANAGED
 python3 -m ensurepip
 install_python_packages
 python3 -m pip install "${base_python_pkgs[@]}"
+
+echo "source /etc/profile" >> /ci/env_vars.sh