CI Linux: add /usr/local/lib to LD_LIBRARY_PATH

+ also to LIBRARY_PATH - this will is done also with -L in configure,
probably

Seems like that /usr/local/lib has never been in LD_LIBRARY_PATH and
somehow it didn't matter so far. But with fluidsynth build, we now get
in CI (== also prevents build):
```
ultragrid_acap_fluidsynth.so: libfluidsynth.so.3: cannot open shared object file: No such file or directory
````

Author: MartinPulec

.github/scripts/Linux/prepare.sh

@@ -1,9 +1,11 @@
 #!/bin/bash -eux
 
 export PKG_CONFIG_PATH=/usr/local/qt/lib/pkgconfig:/usr/local/lib/pkgconfig
+export LIBRARY_PATH=/usr/local/lib:/usr/local/qt/lib
 printf "%b" "\
 CPATH=/usr/local/qt/include\n\
-LIBRARY_PATH=/usr/local/qt/lib\n\
+LIBRARY_PATH=$LIBRARY_PATH\n\
+LD_LIBRARY_PATH=$LIBRARY_PATH\n\
 PKG_CONFIG_PATH=$PKG_CONFIG_PATH\n" >> "$GITHUB_ENV"
 printf "/usr/local/qt/bin\n" >> "$GITHUB_PATH"