nongnu: steam: Fix run.sh for new installs.

• nongnu/packages/steam-client.scm (steam-client)[arguments]: Modify run.sh
  in steam runtime to prefer LD_LIBRARY_PATH.

I just realized that our recent change to avoid re-compiling glibc breaks new installations due to the fact that it relies on a new version of run.sh, but steams runtime still ships with the old version which gets unpacked during the installation.
(Sorry about this, I should definitely make it a point to test from a completely clean slate more often)

This patch modifies the run.sh file inside the runtime archive. Normally steam overwrites such changes, but in this case it works out since re-downloading the file results in the new version. We just needed to modify the initial version so it can run the first time.
Just tested on a clean install and all is working.

@jonsger I tried pushing these changes directly but it says I don't have access. I'm fairly new to working with gpg keys (never used them before contributing to Guix), so I appologize if I'm doing something wrong, but I have my gpg key uploaded to gitlab and also signed this commit (it shows "verified") but it still wouldn't let me push.