Fixes <https://issues.guix.gnu.org/61156>.
* gnu/build/linux-container.scm (container-excursion): Add extra call to
'primitive-fork' and invoke THUNK in the child process.
* tests/containers.scm ("container-excursion"): Remove extra
'primitive-fork' call, now unnecessary.
("container-excursion*, /proc"): New test.