bug#47007: dcb640f02b broke guix environment --container

[Jelle Licht]

Hello guix,

I only tried this on x86_64 guix systems, one with Linux kernel 5.11.2,
and also on a Linux-libre kernel 4.14.223.

Running the equivalent of a `git bisect' starting some months back to
today's master, and with the following test to select bad/good bisect revisions:

`./pre-inst-env guix environment --ad-hoc --container --no-cwd --network hello'

leads me to believe that the following commit broke something.

dcb640f02b * file-systems: 'mount-file-system' preserves source flags for bind 
mounts.

Did this commit expose an existing (but silent) issue, or did it
introduce something new?

For completeness' sake, the output of the guix environment invocation on
'bad' revisions:

--8<---------------cut here---------------start------------->8---
guix environment: error: mount: mount 
"/gnu/store/mmhimfwmmidf09jw1plw3aw1g1zn2nkh-bash-static-5.0.16" on 
"/tmp/guix-directory.Ji7KNW//gnu/store/mmhimfwmmidf09jw1plw3aw1g1zn2nkh-bash-static-5.0.16":
 Operation not permitted
--8<---------------cut here---------------end--------------->8---

I think kozo[m] on irc was hunting the very same issue as well:
http://logs.guix.gnu.org/guix/2021-03-08.log#035557

Thanks,
 - Jelle