fix(build): Preserve FFmpeg libs with -system-libs -hardsubx

The -system-libs mode was overwriting BLD_LINKER and losing the FFmpeg
libraries that -hardsubx adds. This fix preserves the FFmpeg libraries
when both flags are used together.

Also add permissions: contents: write to the workflow to allow
uploading assets to releases.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: cfsmp3

.github/workflows/build_linux_systemlibs.yml

@@ -17,6 +17,9 @@ on:
       - '.github/workflows/build_linux_systemlibs.yml'
       - 'linux/build'
 
+permissions:
+  contents: write
+
 jobs:
   build-systemlibs:
     runs-on: ubuntu-22.04

linux/build

@@ -143,7 +143,12 @@ if [ "$USE_SYSTEM_LIBS" = true ]; then
     $UTF8PROC_COMPAT $PKG_CFLAGS $LEPTONICA_CFLAGS $TESSERACT_CFLAGS $GPAC_CFLAGS"
 
     BLD_SOURCES="../src/ccextractor.c $SRC_CCX $SRC_HASH"
-    BLD_LINKER="$PKG_LIBS -ltesseract -lleptonica -lgpac -lpthread -ldl -lm"
+    # Preserve FFmpeg libraries if -hardsubx was specified
+    FFMPEG_LIBS=""
+    if [ "$HARDSUBX" = true ]; then
+        FFMPEG_LIBS="-lswscale -lavutil -pthread -lavformat -lavcodec -lavfilter -lxcb-shm -lxcb -lX11 -llzma -lswresample"
+    fi
+    BLD_LINKER="$PKG_LIBS -ltesseract -lleptonica -lgpac -lpthread -ldl -lm $FFMPEG_LIBS"
 fi