Allow update_env to incorporate runtime changes to bootstrap.cfg

Perform variable substitution at runtime, rather than at generation
time. This way, if bootstrap.cfg changes after update_env, the new
values there take effect immediately.

Author: Googulator

steps/improve/update_env.sh

@@ -6,7 +6,7 @@
 
 unset GUILE_LOAD_PATH
 
-cat > /steps/env <<- EOF
+cat >> /steps/env <<- 'EOF'
 export PATH=${PREFIX}/bin
 PREFIX=${PREFIX}
 LIBDIR=${LIBDIR}