build: Replace join_paths with the / operator

Since Meson 0.49.0, the / operator on strings is equivalent to calling
join_paths() [1], and the former is less verbose, and easier to read.

[1] https://mesonbuild.com/Reference-manual_functions.html#join_paths

https://github.com/containers/toolbox/pull/1141

data/config/meson.build

@@ -1,4 +1,4 @@
 install_data(
   'toolbox.conf',
-  install_dir: join_paths(get_option('sysconfdir'), 'containers'),
+  install_dir: get_option('sysconfdir') / 'containers',
 )

doc/meson.build

@@ -32,7 +32,7 @@ foreach section, pages: manuals
       command: go_md2man_command,
       input: input,
       install: true,
-      install_dir: join_paths(get_option('mandir'), sectiondir),
+      install_dir: get_option('mandir') / sectiondir,
       output: output,
     )
   endforeach

meson.build

@@ -63,7 +63,7 @@ endif
 
 install_subdir(
   'test',
-  install_dir: join_paths(get_option('datadir'), meson.project_name()),
+  install_dir: get_option('datadir') / meson.project_name(),
   exclude_files: [
     'system/libs/bats-assert/.git',
     'system/libs/bats-assert/.gitignore',