bug#22004: guix-environment-container.sh test fails when building guix on tmpfs

[Ting-Wei Lan]

I ran the test on core-updates branch, but it still failed.

Output in tests/guix-environment-container.log:

+ cat t-guix-environment-31068/mounts
/path/to/a/tmpfs/mountpoint/guix-build
++ wc -l
+ test 1 -eq 4