[nfc] Put pandoc install in configurable location

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@sifive.com>

.github/workflows/pandoc.yml

@@ -12,6 +12,7 @@ on:
 env:
   pandoc-version: 3.1.11.1
   pandoc-crossref-version: v0.3.17.0c
+  pandoc-install-dir: /opt/pandoc
 
 jobs:
   markdown-to-pdf:
@@ -27,10 +28,10 @@ jobs:
       - name: "Install pandoc ${{ env.pandoc-version }}, pandoc-crossref ${{ env.pandoc-crossref-version }}"
         shell: bash
         run: |
-          mkdir -p usr/bin
-          wget https://github.com/jgm/pandoc/releases/download/${{ env.pandoc-version }}/pandoc-${{ env.pandoc-version }}-linux-amd64.tar.gz -O - | tar -zx -C usr/bin --strip-components 2
-          wget https://github.com/lierdakil/pandoc-crossref/releases/download/${{ env.pandoc-crossref-version }}/pandoc-crossref-Linux.tar.xz -O - | tar -Jx -C usr/bin
-          echo "$(pwd)/usr/bin" >> $GITHUB_PATH
+          mkdir -p ${{ env.pandoc-install-dir }}/bin
+          wget https://github.com/jgm/pandoc/releases/download/${{ env.pandoc-version }}/pandoc-${{ env.pandoc-version }}-linux-amd64.tar.gz -O - | tar -zx -C ${{ env.pandoc-install-dir }}/bin --strip-components 2
+          wget https://github.com/lierdakil/pandoc-crossref/releases/download/${{ env.pandoc-crossref-version }}/pandoc-crossref-Linux.tar.xz -O - | tar -Jx -C ${{ env.pandoc-install-dir }}/bin
+          echo "${{ env.pandoc-install-dir }}/bin" >> $GITHUB_PATH
       - name: "Check Formatting"
         shell: bash
         run: |