chore: cache warns about bad v4.8.0-rc1 toolchain (#12623)

Co-authored-by: Scott Morrison <scott.morrison@gmail.com>

Cache/Main.lean

@@ -65,6 +65,10 @@ def leanTarArgs : List String :=
 
 open Cache IO Hashing Requests System in
 def main (args : List String) : IO Unit := do
+  if Lean.versionString == "4.8.0-rc1" && Lean.githash == "b470eb522bfd68ca96938c23f6a1bce79da8a99f" then do
+    println "Unfortunately, you have a broken Lean v4.8.0-rc1 installation."
+    println "Please run `elan toolchain uninstall leanprover/lean4:v4.8.0-rc1` and try again."
+    Process.exit 1
   -- We pass any following arguments to `getHashMemo`,
   -- so we can use the cache on `Archive` or `Counterexamples`.
   let extraRoots := match args with