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

ober

f31a47212f8f08adfec29265744a76ed8b63ff40

diff --git a/.builds/ci.yml b/.builds/ci.yml
index a6cd469..3d24935 100644
--- a/.builds/ci.yml
+++ b/.builds/ci.yml
@@ -21,6 +21,8 @@ artifacts:
 tasks:
 - build-jerboa: |
     cd jerboa
+    sudo fallocate -l 4G /swapfile && sudo chmod 600 /swapfile && sudo mkswap /swapfile && sudo swapon /swapfile
+    export CARGO_BUILD_JOBS=1
     curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | \
       sh -s -- -y --profile minimal --default-toolchain 1.94.1
     source "$HOME/.cargo/env"