[babl] use env instead of bash

[Øyvind Kolås <ok src gnome org>]

commit 0b409e037fe3a4a14507485dfcb916e3a07b46a1
Author: Félix Piédallu <felix piedallu me>
Date:   Sat Dec 2 00:50:41 2017 +0100

    use env instead of bash

 docs/meson.build |    5 +++--
 1 files changed, 3 insertions(+), 2 deletions(-)
---
diff --git a/docs/meson.build b/docs/meson.build
index 3a5115b..3cef2d1 100644
--- a/docs/meson.build
+++ b/docs/meson.build
@@ -18,8 +18,9 @@ index_static_html = configure_file(
 index_html_tmp = custom_target('index.html.tmp',
   input : [ babl_html_dump, ],
   output: [ 'index.html.tmp', ],
-  command: [ 'bash', '-c',
-    'BABL_PATH="'+ join_paths(meson.build_root(), 'extensions') +'" '+ babl_html_dump.full_path()
+  command: [
+    'env', 'BABL_PATH='+ join_paths(meson.build_root(), 'extensions'),
+    babl_html_dump
   ],
   capture: true,
 )