fix(ci): add library paths for lzma-sys on self-hosted runner

The six-peer-regression job compiles freenet from a separate workspace
(river) which doesn't benefit from cached build artifacts. On Arch Linux
runners, the linker needs explicit paths to find liblzma.

[AI-assisted - Claude]

Author: sanity

.github/workflows/ci.yml

@@ -139,6 +139,9 @@ jobs:
 
     env:
       FREENET_LOG: error
+      # Ensure lzma-sys can find liblzma on Arch Linux (self-hosted runner)
+      LIBRARY_PATH: /usr/lib
+      PKG_CONFIG_PATH: /usr/lib/pkgconfig
 
     steps:
       - uses: actions/checkout@v6