terraform-lint: Bump to ubuntu-24.04

Motivated by <nextstrain/.github#113>

Since Terraform is no longer available, we have to install it as the
first step in the job. Installation commands copied from
<https://developer.hashicorp.com/terraform/install#linux>

.github/workflows/terraform-lint.yml

@@ -21,8 +21,18 @@ jobs:
       matrix:
         env:
           - env/production
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-24.04
     steps:
+      - name: Install Terraform
+        run: >
+          wget -O - https://apt.releases.hashicorp.com/gpg
+          | sudo gpg --dearmor -o /usr/share/keyrings/hashicorp-archive-keyring.gpg
+
+          echo "deb [arch=$(dpkg --print-architecture) signed-by=/usr/share/keyrings/hashicorp-archive-keyring.gpg] https://apt.releases.hashicorp.com $(lsb_release -cs) main"
+          | sudo tee /etc/apt/sources.list.d/hashicorp.list
+
+          sudo apt update && sudo apt install terraform
+
       - uses: actions/checkout@v4
 
       - run: terraform init -backend=false