Avoid opening the same file twice

Instead, use a subshell and redirect its stdout to the relevant file.

Signed-off-by: John Keeping <john@keeping.me.uk>
Add an "exec" instruction

This allows a command to be executed as part of the integration rebuild
process, which may be useful, for example, to execute a script to update
submodules to their own suitable integration versions.

Signed-off-by: John Keeping <john@keeping.me.uk>