[REVERT] CI: Pin RustyHermit for xtask

Author: mkroening

.github/workflows/aarch64.yml

@@ -28,6 +28,7 @@ jobs:
         with:
           repository: hermitcore/rusty-hermit
           submodules: true
+          ref: df00caebb623f5b362ea4fa0eeecf9ae2cf79d8a
       - name: Remove libhermit-rs submodule
         run: git rm -r libhermit-rs
       - name: Checkout libhermit-rs

.github/workflows/x86.yml

@@ -44,6 +44,7 @@ jobs:
         with:
           repository: hermitcore/rusty-hermit
           submodules: true
+          ref: df00caebb623f5b362ea4fa0eeecf9ae2cf79d8a
       - name: Remove libhermit-rs submodule
         run: git rm -r libhermit-rs
       - name: Checkout libhermit-rs