refactor: lake: restrict cache fetch to leanprover* (#5642)

Lake will now only automatically fetch Reservoir build caches for
package in the the `leanprover` and `leanprover-community`
organizations. We are not planning to expand the Reservoir build cache
to other packages until farther in the future.

src/lake/Lake/Build/Package.lean

@@ -44,9 +44,9 @@ def Package.maybeFetchBuildCache (self : Package) : FetchM (BuildJob Bool) := do
   let shouldFetch :=
     (← getTryCache) &&
     (self.preferReleaseBuild || -- GitHub release
-      !(self.scope.isEmpty -- no Reservoir
-        || (← getElanToolchain).isEmpty
-        || (← self.buildDir.pathExists)))
+      ((self.scope == "leanprover" || self.scope == "leanprover-community")
+        && !(← getElanToolchain).isEmpty
+        && !(← self.buildDir.pathExists))) -- Reservoir
   if shouldFetch then
     self.optBuildCache.fetch
   else