feat: GitHub cloud releases do not clobber prebuilt artifacts (#6218)

This PR makes Lake no longer automatically fetch GitHub cloud releases
if the package build directory is already present (mirroring the
behavior of the Reservoir cache). This prevents the cache from
clobbering existing prebuilt artifacts. Users can still manually fetch
the cache and clobber the build directory by running `lake build
<pkg>:release`.

src/lake/Lake/Build/Package.lean

@@ -47,10 +47,10 @@ def Package.optBuildCacheFacetConfig : PackageFacetConfig optBuildCacheFacet :=
 def Package.maybeFetchBuildCache (self : Package) : FetchM (BuildJob Bool) := do
   let shouldFetch :=
     (← getTryCache) &&
+    !(← self.buildDir.pathExists) && -- do not automatically clobber prebuilt artifacts
     (self.preferReleaseBuild || -- GitHub release
       ((self.scope == "leanprover" || self.scope == "leanprover-community")
-        && !(← getElanToolchain).isEmpty
-        && !(← self.buildDir.pathExists))) -- Reservoir
+        && !(← getElanToolchain).isEmpty)) -- Reservoir
   if shouldFetch then
     self.optBuildCache.fetch
   else