runqemu: Do not perturb script environment

Instead of changing the script environment to affect the child
processes, make a copy of the environment with modifications and pass
that to subprocess.

Specifically, when dri rendering is enabled, LD_PRELOAD was being passed
to all processes created by the script which resulted in other commands
(e.g. stty) exiting with a failure like:

 /bin/sh: symbol lookup error: sysroots-uninative/x86_64-linux/lib/librt.so.1: undefined symbol: __libc_unwind_link_get, version GLIBC_PRIVATE

Making a copy of the environment fixes this because the LD_PRELOAD is
now only passed to qemu itself.

Signed-off-by: Joshua Watt <JPEWhacker@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>