file: fix the way path is written to environment-setup.d

file: fix the way path is written to environment-setup.d

$OECORE_NATIVE_SYSROOT is not escaped, thus it is expanded to an empty string.
This happens to work because we have ${datadir} which in this context is
relocated to the buildtools sysroot after installation of buildtools.

If the $ before OECORE_NATIVE_SYSROOT is escaped, it will be saved to file.sh
as intended, but MAGIC will point to the wrong location because both
OECORE_NATIVE_SYSROOT and datadir resolve to sysroot which doubles the path
like so:
/path/to/buildtools/sysroots/x86_64-xesdk-linux/path/to/buildtools/sysroots/x86_64-xesdk-linux/usr/share/misc/magic.mgc
which does not exist. So, removing reference to OECORE_NATIVE_SYSROOT.

Signed-off-by: Oleksandr Hnatiuk <ohnatiuk@cisco.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>