cmd/initContainer: Simplify removing the user's password

[debarshiray]

It's one less invocation of an external command, which is good because
spawning a new process is generally expensive.

One positive side-effect of this is that on some Active Directory
set-ups, the entry point no longer fails with:

  Error: failed to remove password for user [email protected]: failed
      to invoke passwd(1)

... because of:

  # passwd --delete [email protected]
  passwd: Libuser error at line: 210 - name contains invalid char `@'.

This is purely an accident, and isn't meant to be an intential change to
support Active Directory. Tools like useradd(8) and usermod(8) from
Shadow aren't meant to work with Active Directory users, and, hence, it
can still break in other ways. For that, one option is to expose $USER
from the host operating system to the Toolbx container through a Varlink
interface that can be used by nss-systemd inside the container.

Based on an idea from Si.

#585

[softwarefactory-project-zuul]

Build failed.
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[softwarefactory-project-zuul]

Build failed.
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[softwarefactory-project-zuul]

Build failed.
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[softwarefactory-project-zuul]

Build failed.
https://softwarefactory-project.io/zuul/t/local/buildset/324eed88ed8544c28849d7cbf56ab9af

✔️ unit-test SUCCESS in 8m 26s
✔️ unit-test-migration-path-for-coreos-toolbox SUCCESS in 3m 09s
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❌ system-test-fedora-rawhide FAILURE in 33m 58s
✔️ system-test-fedora-38 SUCCESS in 30m 31s
✔️ system-test-fedora-37 SUCCESS in 29m 20s

[debarshiray]

There are still some test failures on Fedora Rawhide. For example:

fedora-rawhide | not ok 3 help: Run command 'help' in 145ms
fedora-rawhide | # (from function `assert_line' in file test/system/libs/bats-assert/src/assert.bash, line 479,
fedora-rawhide | #  in test file test/system/002-help.bats, line 45)
fedora-rawhide | #   `assert_line --index 0 --partial "toolbox(1)"' failed
fedora-rawhide | # /usr/bin/man
fedora-rawhide | #
fedora-rawhide | # -- line does not contain substring --
fedora-rawhide | # index     : 0
fedora-rawhide | # substring : toolbox(1)
fedora-rawhide | # line      : troff:<standard input>:33: warning: cannot select font 'C'
fedora-rawhide | # --

I believe these are because of changes in various other components in Fedora 39, which we need to track down one by one and work out a fix.

In the mean time, I am going to temporarily override these failures.