imghelper: hoist AWS vars into global environment

When secure boot signing was refactored in 310cadd, the code that
exports AWS credential variables into the global environment was moved
into a bash function.

When used in a bash function, the `declare` keyword makes the declared
symbol local, unless the -g option is also supllied. This adds the -g
option to export the variables into the caller's environment as
intended.

twoliter/embedded/imghelper

@@ -414,7 +414,7 @@ sbsetup_aws_profile() {
     val="${var,,}"
     val="${HOME}/.aws/${val//_/-}.env"
     [[ -s "${val}" ]] || continue
-    declare -x "${var}=$(cat "${val}")"
+    declare -g -x "${var}=$(cat "${val}")"
   done
   # Verify that AWS credentials are functional.
   aws sts get-caller-identity