build: find system luv on ubuntu

Also use the system luv in CI for the with-external-deps job.

.github/workflows/test.yml

@@ -383,10 +383,10 @@ jobs:
             lua-filesystem \
             lua-lpeg \
             lua-mpack \
-            luajit
+            luajit \
+            lua-luv-dev
             # libtree-sitter-dev \
-            # libvterm-dev \
-            # lua-luv-dev
+            # libvterm-dev
 
             # Remove comments from packages once we start using these external
             # dependencies.
@@ -400,7 +400,6 @@ jobs:
           # bundled versions for these with the hopes of being able to remove them
           # later on.
           cmake -S cmake.deps -B .deps -G Ninja -D USE_BUNDLED=OFF \
-            -D USE_BUNDLED_LUV=ON \
             -D USE_BUNDLED_LIBVTERM=ON \
             -D USE_BUNDLED_TS=ON
           cmake --build .deps

cmake/FindLibluv.cmake

@@ -1,5 +1,14 @@
 find_path(LIBLUV_INCLUDE_DIR luv/luv.h)
-find_library(LIBLUV_LIBRARY NAMES luv_a luv libluv_a luv.so)
+find_library(LIBLUV_LIBRARY NAMES luv_a luv libluv_a)
+
+# Ubuntu-specific workaround to find system paths
+function(ubuntu)
+  set(CMAKE_FIND_LIBRARY_PREFIXES "")
+  find_path(LIBLUV_INCLUDE_DIR luv/luv.h PATH_SUFFIXES lua5.1)
+  find_library(LIBLUV_LIBRARY NAMES luv PATH_SUFFIXES lua/5.1)
+endfunction()
+ubuntu()
+
 find_package_handle_standard_args(Libluv DEFAULT_MSG
   LIBLUV_LIBRARY LIBLUV_INCLUDE_DIR)
 mark_as_advanced(LIBLUV_INCLUDE_DIR LIBLUV_LIBRARY)