nixos/kexec-boot: use dirname of script to resolve bzImage and initrd.gz

This will allow invoking the `kexec-boot` script without `cd`-ing into
its folder first.

nixos/modules/installer/kexec/kexec-boot.nix

@@ -27,8 +27,9 @@
             echo "kexec not found: please install kexec-tools" 2>&1
             exit 1
           fi
-          kexec --load ./bzImage \
-            --initrd=./initrd.gz \
+          SCRIPT_DIR=$( cd -- "$( dirname -- "''${BASH_SOURCE[0]}" )" &> /dev/null && pwd )
+          kexec --load ''${SCRIPT_DIR}/bzImage \
+            --initrd=''${SCRIPT_DIR}/initrd.gz \
             --command-line "init=${config.system.build.toplevel}/init ${toString config.boot.kernelParams}"
           kexec -e
         ''; in