ci(olean_report): use lake env instead of lake exec to invoke cache binary (#38712)

The fix in #38706 was broken.

`lake exec` treats its first argument as a Lake target specifier, so it rejects absolute paths containing multiple slashes. `lake env <cmd>` sets up LEAN_PATH and other Lake environment variables for the current package, then execs the command directly — which is what we need.

Prepared with Claude code.

Author: bryangingechen

.github/workflows/olean_report.yaml

@@ -174,7 +174,7 @@ jobs:
         continue-on-error: true
         run: |
           cd pr-branch
-          lake exec "$CACHE_BIN" get
+          lake env "$CACHE_BIN" get
 
       - name: Check PR head oleans
         id: pr_oleans_check
@@ -195,7 +195,7 @@ jobs:
         continue-on-error: true
         run: |
           cd base
-          lake exec "$CACHE_BIN" get
+          lake env "$CACHE_BIN" get
 
       - name: Generate olean report
         if: steps.check_trigger.outputs.triggered == 'true'