pkgsMusl.libical: fix build by disabling tests

We can't support running tests on Musl until TZDIR support is added.

pkgs/development/libraries/libical/default.nix

···
 
   # Using install check so we do not have to manually set
   # LD_LIBRARY_PATH and GI_TYPELIB_PATH variables
-  doInstallCheck = true;
+  # Musl does not support TZDIR.
+  doInstallCheck = !stdenv.hostPlatform.isMusl;
   enableParallelChecking = false;
   preInstallCheck = if stdenv.isDarwin then ''
     for testexe in $(find ./src/test -maxdepth 1 -type f -executable); do