fix(kexec): zstd file extension is zst

https://datatracker.ietf.org/doc/html/rfc8478

Author: sedlund

src/nixos-anywhere.sh

@@ -677,7 +677,7 @@ TMPDIR=/root/kexec setsid --wait ${maybeSudo} /root/kexec/kexec/run --kexec-extr
     tarDecomp="--gzip"
   elif [[ ${kexecUrl} =~ \.tar\.xz$ ]]; then
     tarDecomp="--xz"
-  elif [[ ${kexecUrl} =~ \.tar\.zstd$ ]]; then
+  elif [[ ${kexecUrl} =~ \.tar\.zst$ ]]; then
     tarDecomp="--zstd"
   elif [[ ${kexecUrl} =~ \.tar$ ]]; then
     tarDecomp=""