runtime: Reset PATH in the container
The PATH from outside the container doesn't really make sense inside
the container, because the directory hierarchy could be totally different
(particularly on unusual host systems like GoboLinux and Exherbo).
Resetting it also makes sure we don't pick up local installations in
~/bin or ~/.local/bin, which could have dependencies that are
unsatisfied in the container.
Conversely, the PATH from the host might not include everything we need
in the container. If the host system is an OS that only supports
merged-/usr use, like Arch Linux, then it might not have both /usr/bin
and /bin in the PATH - but if the container is not merged /usr, like a
scout SDK sysroot, then we'll need both.
This approximately matches Flatpak's behaviour: by default it resets
PATH to /app/bin:/usr/bin (because it puts app-specific content in /app
and can assume that its runtimes are always merged-/usr).
Signed-off-by:
Simon McVittie <smcv@collabora.com>