fix: make TEKTON_REGISTRY_OVERRIDE apply to all images

[sybernatus]

Changes

Previously the override only rewrote images already present in the per-component IMAGE_* env var map, leaving the defaults baked into component manifests untouched, and existing TektonInstallerSets never refreshed when only the registry override changed on an existing install.

TEKTON_REGISTRY_OVERRIDE was introduced in #2863 (docs in #2865, originally requested in #2684 / #2855) but only rewrote images that already had a matching per-image IMAGE_* env var set. This is exactly the gap reported in #3270 ("Rewrite image registry Not working", still open) and previously in #2873 ("only one of the resulting containers correctly refers to the override registry, all other pods still refer to the default location").

This PR makes TEKTON_REGISTRY_OVERRIDE alone sufficient to rewrite every image, and fixes existing installs to pick up the change automatically:

• replaceContainerImages (Deployment/StatefulSet/Job containers), replaceStepsImages (Task/ClusterTask steps) and replaceStepActionImages (StepAction) now fall back to rewriting the registry host of the image already present in the manifest when no per-image env var matches it, instead of leaving it untouched. Tekton variable substitutions ($(params.foo)) are explicitly excluded from this fallback since they aren't literal image references.
• The TektonInstallerSet spec-hash (specHashInput) now includes TEKTON_REGISTRY_OVERRIDE, so changing it on an existing install triggers a refresh of the installer sets on the next reconcile instead of requiring a manual kubectl delete tektoninstallerset.
• docs/AirGapImageConfiguration.md updated to reflect the new behavior.

Note: #3270 also reports that some arg:-based image overrides (IMAGE_TRIGGERS_ARG__EL_IMAGE, IMAGE_PIPELINES_ARG__WORKINGDIRINIT_IMAGE) don't apply even when set explicitly. That's a separate, pre-existing bug in replaceContainersArgsImage unrelated to the registry-override fallback and isn't addressed here.

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Release Notes

fix: TEKTON_REGISTRY_OVERRIDE now applies to every image, including component defaults with no matching IMAGE_* env var, and existing installs refresh automatically when the value changes.

[tekton-robot]

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 25.61%. Comparing base (a5da4e5) to head (721b3e4).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3754      +/-   ##
==========================================
+ Coverage   25.55%   25.61%   +0.06%     
==========================================
  Files         448      448              
  Lines       23317    23330      +13     
==========================================
+ Hits         5958     5976      +18     
+ Misses      16674    16671       -3     
+ Partials      685      683       -2     

Flag	Coverage Δ
unit-tests	25.61% <ø> (+0.06%)	⬆️

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[anithapriyanatarajan]

/lgtm