Limit Nix build parallelism to reduce memory usage

.forgejo/workflows/deploy.yml

@@ -10,6 +10,8 @@ jobs:
     runs-on: nixos
     env:
       PATH: /run/current-system/sw/bin
+      NIX_BUILD_CORES: 1
+      NIX_MAX_JOBS: 1
     steps:
       - name: Clone repo
         run: |
@@ -20,11 +22,11 @@ jobs:
           echo "WORKDIR=$WORKDIR" >> $GITHUB_ENV
 
       - name: Build website with Nix
-        run: nix build .#website --no-write-lock-file
+        run: nix build .#website --no-write-lock-file --cores 1 --max-jobs 1
         working-directory: ${{ env.WORKDIR }}
 
       - name: Build speaker profile PDF
-        run: nix build .#speaker-profile --no-write-lock-file
+        run: nix build .#speaker-profile --no-write-lock-file --cores 1 --max-jobs 1
         working-directory: ${{ env.WORKDIR }}
 
       - name: Copy build artifacts