generate-reprepro-codename: lock access to conf/distributions

With bad timing we might run into something like this:

| + grep -q '^Codename: release-mr3.2$' /srv/repository/release/release-mr3.2/conf/distributions
| + echo 'Codename/repository release-mr3.2 exists already, ignoring request to add again.'
| Codename/repository release-mr3.2 exists already, ignoring request to add again.

Use flock(1) from util-linux to lock access to the
reprepro configuration file.

Related to commit 14f3559a
Closes: MT#4751
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