Add quotes to prevent potential word splittings

[zzmic]

As the title suggests, this PR includes the commit that adds quotes to several commands in scripts/download_gfx.sh to prevent potential word splittings (FYI, https://www.shellcheck.net/wiki/SC2086). In particular, the previous line of rm -rf /tmp/${GFX_DRIVER} is concerning, since ${GFX_DRIVER} is unquoted, (1) if it contains whitespaces, it would be split into multiple arguments to rm -rf, and (2) if it is empty or resolves unexpectedly, the command becomes rm -rf /tmp/, which is even more catastrophic. Other quotes are added more for the sake of consistency, even though they do address the valid concern of word splitting (but they are not as concerning as rm -rf /tmp/${GFX_DRIVER}).