build: preserve correct mode when generating files via jinja2

build: preserve correct mode when generating files via jinja2

When using "capture : true" in custom_target()s the mode of the source
file is not preserved when the generated file is not installed and so
needs to be tweaked manually. Switch from output capture to creating the
target file and copy the permissions from the input file.

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>