ci: bump riscv64-linux timeout from 6 hours to 7 hours

GitHub is apparently very bad at arithmetic and so will cancel jobs that pass
the 5 hours mark, even if they're nowhere near the 6 hours timeout. So add an
hour to assist GitHub in this very difficult task.

Author: alexrp

.github/workflows/ci.yaml

@@ -52,7 +52,7 @@ jobs:
         run: sh ci/aarch64-linux-release.sh
   riscv64-linux-debug:
     if: github.event_name == 'push'
-    timeout-minutes: 360
+    timeout-minutes: 420
     runs-on: [self-hosted, Linux, riscv64]
     steps:
       - name: Checkout
@@ -61,7 +61,7 @@ jobs:
         run: sh ci/riscv64-linux-debug.sh
   riscv64-linux-release:
     if: github.event_name == 'push'
-    timeout-minutes: 360
+    timeout-minutes: 420
     runs-on: [self-hosted, Linux, riscv64]
     steps:
       - name: Checkout