Changes

jobs/ttcn3-testsuites-testenv: fix rm apt partial

Fix that testenv jobs may fail to remove the cache at the beginning:

  rm: cannot remove '_cache/podman/var-lib-apt-debian-trixie/lists/partial': Permission denied

This happens when the partial directory still exists:

  $ ls -lah ~/jenkins/workspace/ttcn3-bsc-test-sccplite/_cache/podman/var-lib-apt-debian-trixie/lists
  total 12K
  drwxr-xr-x 3 osmocom-build osmocom-build 4.0K Apr 27 08:48 .
  drwxr-xr-x 3 osmocom-build osmocom-build 4.0K Apr 26 08:48 ..
  drwx------ 2        100041 osmocom-build 4.0K Apr 26 08:48 partial

Which in turn is the result of "apt update" failing to download files in
an earlier job:

  E: Failed to fetch https://downloads.osmocom.org/packages/osmocom:/nightly/Debian_13/./Packages.gz  File has unexpected size (210767 != 210491). Mirror sync in progress? [IP: 148.251.236.141 443]
     Hashes of expected file:
      - Filesize:210491 [weak]
      - SHA256:a0cdea838b724880f455d68eb433f6331a7d4113c2eb677ca2f7f6dbdf9a72e1
      - SHA1:71780896257ca550e51b95e77fee4c63fcc9980c [weak]
      - MD5Sum:531a2d49d4e9f3ff121ca6aa5be602fc [weak]
     Release file created at: Sun, 26 Apr 2026 08:48:04 +0000
  E: Some index files failed to download. They have been ignored, or old ones used instead.

Change-Id: I35b9e42b754805b42f0a868a5d0c20c1f1c01731