CI: drop github context from ccache-setup input description

The read-only input description embedded `${{ github.event_name ==
'pull_request' }}` as example text. GitHub validates ${{ }} expressions in
an action's input definitions at manifest-load time, where the github
context is not available, so the action failed to load ("Unrecognized
named-value: 'github'", action.yml line 27) and every ccache-setup
consumer died at "Set up ccache" (build library, make check, Compiler
test, Multi-arch test, ...). Describe the expression in prose instead of
embedding it as a live ${{ }} template; the github.* references that
remain are in step with:/run: blocks, where the context is available.

.github/actions/ccache-setup/action.yml

@@ -26,9 +26,10 @@ inputs:
   read-only:
     description: >
       When 'true', restore the cache but do NOT save it (no post-job
-      upload). Pass `${{ github.event_name == 'pull_request' }}` so PR runs
-      consume the shared cache read-only - no per-PR entries, no churn -
-      while scheduled/push runs (read-only false) refresh it.
+      upload). Callers should set this to the result of the expression
+      github.event_name == 'pull_request' so PR runs consume the shared
+      cache read-only - no per-PR entries, no churn - while scheduled/push
+      runs (read-only false) refresh it.
     required: false
     default: 'false'