Correct computing ELN image name from TMT distro variable

Exucuting TMT plan for fedora-eln Packit target failed:

    13:55:51 tmt command line: /usr/bin/tmt --root . -c arch=x86_64 -c distro=fedora-eln [...]
    [...]
    [...] /var/tmp/ci-dnf-stack/container-test build \
    --base $( echo "fedora-eln" | tr '"'"'-'"'"' '"'"':'"'"') \
    --container-arg="--env=COPR=packit/rpm-software-management-createrepo_c-476" \
    --container-arg="--env=COPR_RPMS=createrepo_c-devel-1.2.2-1.20260306120418561713.pr476.9.g5ee07e4.eln155.x86_64 createrepo_c-debuginfo-1.2.2-1.20260306120418561713.pr476.9.g5ee07e4.eln155.x86_64 python3-createrepo_c-debuginfo-1.2.2-1.20260306120418561713.pr476.9.g5ee07e4.eln155.x86_64 createrepo_c-libs-debuginfo-1.2.2-1.20260306120418561713.pr476.9.g5ee07e4.eln155.x86_64 createrepo_c-1.2.2-1.20260306120418561713.pr476.9.g5ee07e4.eln155.x86_64 createrepo_c-debugsource-1.2.2-1.20260306120418561713.pr476.9.g5ee07e4.eln155.x86_64 createrepo_c-libs-1.2.2-1.20260306120418561713.pr476.9.g5ee07e4.eln155.x86_64 python3-createrepo_c-1.2.2-1.20260306120418561713.pr476.9.g5ee07e4.eln155.x86_64"
    '
    13:56:24             cmd:
		    set -eo pipefail; /var/tmp/ci-dnf-stack/container-test build \
		    --base $( echo "fedora-eln" | tr '-' ':') \
		    --container-arg="--env=COPR=packit/rpm-software-management-createrepo_c-476" \
		    --container-arg="--env=COPR_RPMS=createrepo_c-devel-1.2.2-1.20260306120418561713.pr476.9.g5ee07e4.eln155.x86_64 createrepo_c-debuginfo-1.2.2-1.20260306120418561713.pr476.9.g5ee07e4.eln155.x86_64 python3-createrepo_c-debuginfo-1.2.2-1.20260306120418561713.pr476.9.g5ee07e4.eln155.x86_64 createrepo_c-libs-debuginfo-1.2.2-1.20260306120418561713.pr476.9.g5ee07e4.eln155.x86_64 createrepo_c-1.2.2-1.20260306120418561713.pr476.9.g5ee07e4.eln155.x86_64 createrepo_c-debugsource-1.2.2-1.20260306120418561713.pr476.9.g5ee07e4.eln155.x86_64 createrepo_c-libs-1.2.2-1.20260306120418561713.pr476.9.g5ee07e4.eln155.x86_64 python3-createrepo_c-1.2.2-1.20260306120418561713.pr476.9.g5ee07e4.eln155.x86_64"
    13:56:24         environment
    13:56:24             stdout: STEP 1/18: FROM fedora:eln
    13:56:24             stderr: Resolved "fedora" as an alias (/etc/containers/registries.conf.d/000-shortnames.conf)
    13:56:24             stderr: Trying to pull registry.fedoraproject.org/fedora:eln...
    13:56:24             stderr: Error: creating build container: unable to copy from source docker://registry.fedoraproject.org/fedora:eln: initializing source docker://registry.fedoraproject.org/fedora:eln: reading manifest eln in registry.fedoraproject.org/fedora: manifest unknown
    13:56:24             stderr: Error: Failed to build the container.

The cause was that while normal fedora-rawhide target is known as
"fedora:rawhide" image in the image registry, fedora-eln target has
"eln:latest" image.

This patch special-case this specially named image.

Author: ppisar

plans/main.fmf

@@ -26,6 +26,7 @@ prepare:
       how: shell
       script: |
         /var/tmp/ci-dnf-stack/container-test build \
-        --base $( echo "$@distro" | tr '-' ':') \
+        --base $( if test "$@distro" == "fedora-eln"; then echo "eln:latest"; \
+                  else echo "$@distro" | tr '-' ':'; fi ) \
         --container-arg="--env=COPR=$PACKIT_COPR_PROJECT" \
         --container-arg="--env=COPR_RPMS=$PACKIT_COPR_RPMS"