Use yuzu-mirror copies for MerryMage repos

These repositories, unfortunately, do not exist anymore.

.github/workflows/aarch64.yml

@@ -134,7 +134,7 @@ jobs:
       - name: Checkout ext-boost repo
         uses: actions/checkout@v2
         with:
-          repository: MerryMage/ext-boost
+          repository: yuzu-mirror/ext-boost
           path: externals/ext-boost
 
       - name: Setup msvc-arm64 environment

.github/workflows/multiarch.yml

@@ -25,7 +25,7 @@ jobs:
       - name: Checkout ext-boost repo
         uses: actions/checkout@v2
         with:
-          repository: MerryMage/ext-boost
+          repository: yuzu-mirror/ext-boost
           path: externals/ext-boost
 
       - name: Configure CMake

.github/workflows/riscv64.yml

@@ -35,7 +35,7 @@ jobs:
       - name: Checkout ext-boost repo
         uses: actions/checkout@v2
         with:
-          repository: MerryMage/ext-boost
+          repository: yuzu-mirror/ext-boost
           path: externals/ext-boost
 
       - name: Configure CMake for RISCV64 (Linux)

.github/workflows/x86-64.yml

@@ -33,14 +33,14 @@ jobs:
       - name: Checkout ext-boost repo
         uses: actions/checkout@v2
         with:
-          repository: MerryMage/ext-boost
+          repository: yuzu-mirror/ext-boost
           path: externals/ext-boost
 
       - name: Checkout unicorn repo
         if: ${{matrix.os == 'ubuntu-latest' || matrix.os == 'macos-latest'}}
         uses: actions/checkout@v2
         with:
-          repository: MerryMage/unicorn
+          repository: yuzu-mirror/unicorn
           path: externals/unicorn
 
       - name: Build unicorn