Add 4GB swap + limit cargo to 1 job (avoid OOM on 2GB sr.ht VMs)

ober

bcb2f2e1212514934d5da0a4c5e6703b165085d1

diff --git a/.build.yml b/.build.yml
index 37826e7..75b8675 100644
--- a/.build.yml
+++ b/.build.yml
@@ -19,6 +19,8 @@ sources:
 tasks:
   - build-jerboa-tools: |
       cd jerboa
+      sudo fallocate -l 4G /swapfile && sudo chmod 600 /swapfile && sudo mkswap /swapfile && sudo swapon /swapfile
+      export CARGO_BUILD_JOBS=1
       # Build the multicall Jerboa tool artifact used by jerboa-shell.
       curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | \
         sh -s -- -y --profile minimal --default-toolchain 1.94.1