xtask: Add distro-aware bind-storage-ro support for tmt tests

CentOS 9 lacks systemd.extra-unit.* support which is required for
--bind-storage-ro to work with bcvk. This was causing test failures
on centos-9 while working fine on Fedora.

Change the approach so tests express intent via `extra.try_bind_storage: true`
metadata, and xtask handles the details:

- Detect distro by running the container image and parsing os-release
- Pass distro to tmt via --context=distro=<id>-<version>
- Only add --bind-storage-ro when test wants it AND distro supports it
- When bind storage is available, also set BOOTC_upgrade_image env var
- Tests can detect missing $env.BOOTC_upgrade_image and fall back to
  building the upgrade image locally

Add --upgrade-image CLI option to specify the upgrade image path,
replacing the old --env=BOOTC_upgrade_image approach.

Extract magic values to clear const declarations at the top of the file
for better maintainability.

Assisted-by: Claude Code (Sonnet 4.5)
Signed-off-by: Colin Walters <walters@verbum.org>

Author: cgwalters

Justfile

@@ -119,7 +119,7 @@ _build-upgrade-image:
 # Assume the localhost/bootc-integration image is up to date, and just run tests.
 # Useful for iterating on tests quickly.
 test-tmt-nobuild *ARGS:
-    cargo xtask run-tmt --env=BOOTC_variant={{variant}} --env=BOOTC_upgrade_image={{integration_upgrade_img}} {{integration_img}} {{ARGS}}
+    cargo xtask run-tmt --env=BOOTC_variant={{variant}} --upgrade-image={{integration_upgrade_img}} {{integration_img}} {{ARGS}}
 
 # Cleanup all test VMs created by tmt tests
 tmt-vm-cleanup: