Thanks, that's good advice. There are lower-numbered gfx* files in there. 900, 902, 904, 906. No 1030 or 1100. Same after reinstalling.
Looks like these files are actually provided by the libclc-15
package. libclc-16 has the same set of files. Even libclc-17 from sid has the same files. So I guess upgrading to testing/unstable wouldn't help.
apt-file search gfx1100-amdgcn-mesa-mesa3d.bc
yields no results, so I guess I need to go outside of the Debian repos. I'll try the AMD package tonight.
Can you explain more about your workflow? Do the Nix packages have their own isolated dependency resolution? How does it work when Debian packages depend on a library you get from Nix, or vice-versa?