Backport of client/logmon: acquire executable in init block into release/1.2.x

[hc-github-team-nomad-core]

Backport

This PR is auto-generated from #14297 to be assessed for backporting due to the inclusion of the label backport/1.2.x.

The below text is copied from the body of the original PR.

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This PR causes the logmon task runner to acquire the path of binary of the
Nomad executable file in an init block, so as to almost certainly acquire
the name while the nomad file still exists.

This is an attempt at fixing the case where a deleted Nomad file
(e.g. during upgrade) may be getting renamed with a suffix before the deletion
(maybe SELinux specific?).

If this doesn't work, as a last resort we can literally just trim the mystery string.

Fixes: #14079

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