fix(ci): add x-skip-install-test flag for first-build bootstrapping

e2e-install reads the live gh-pages Flatpak index, which is updated by
update-index.yml only after a fully successful build run. On a first-time
app build the app is not yet in the index, causing flatpak install to fail
with 'Nothing matches <app-id> in remote testhub', which fails the build,
which prevents update-index from running — a circular dependency.

Add x-skip-install-test: true support in e2e-install's arch-check step
(manifest.yaml apps only) so new apps can pass CI on their first build.
Set the flag on io.github.DenysMb.Kontainer for bootstrapping; remove
after the index is updated.

Also clean up stale test comment in manifest.yaml.

Assisted-by: Claude Sonnet 4.6 via OpenCode

Author: castrojo

.github/workflows/build.yml

@@ -1062,6 +1062,11 @@ jobs:
               echo "==> Skipping ${{ matrix.app }} install-test (x-disabled: true)"
               echo "skip=true" >> "$GITHUB_OUTPUT"
             fi
+            SKIP_INSTALL=$(yq '.["x-skip-install-test"] // ""' "flatpaks/${{ matrix.app }}/manifest.yaml")
+            if [[ "${SKIP_INSTALL}" == "true" ]]; then
+              echo "==> Skipping ${{ matrix.app }} install-test (x-skip-install-test: true)"
+              echo "skip=true" >> "$GITHUB_OUTPUT"
+            fi
           fi
 
       - name: Read app-id from manifest

flatpaks/io.github.DenysMb.Kontainer/manifest.yaml

@@ -1,5 +1,4 @@
 # Auto-imported from flatpak-tracker issue #528 — do not edit manually
-# test: trigger build to verify lowercase-fix (will be squashed on merge)
 runtime: org.kde.Platform
 runtime-version: '6.10'
 sdk: org.kde.Sdk
@@ -28,3 +27,4 @@ x-version: '1.4.1'
 x-arches:
 - x86_64
 x-skip-chunkah: true
+x-skip-install-test: true