shell: fix colmena run

Somehow SHELL variable from host was still kept in pure shell.

This should prevent colmena deploys failing when host is built against
libc newer than the one in our fhsuserenv.

Change-Id: Ia98e35e71d8d3a0ae2920a18ff2fe6a2e8ed2d53
Reviewed-on: https://gerrit.hackerspace.pl/c/hscloud/+/1910
Reviewed-by: q3k <q3k@hackerspace.pl>
diff --git a/shell.nix b/shell.nix
index 5d155c6..ea96061 100644
--- a/shell.nix
+++ b/shell.nix
@@ -20,6 +20,7 @@
     export NIX_SSL_CERT_FILE="${cacert}/etc/ssl/certs/ca-bundle.crt"
     export SSL_CERT_FILE="${cacert}/etc/ssl/certs/ca-bundle.crt"
     export FONTCONFIG_FILE="${fontconfig}"
+    export SHELL="${pkgs.bash}/bin/bash"
     export HSCLOUD_NIX=yes
 
     source ${toString ./.}/env.sh