Further secure eval call in ConfigStaticManifest

It shouldnt be possible to get ConfigStaticManifest to parse
a user uploaded file, and if you could it shouldnt be possible
to form PHP that token_get_all could parse which would end
up executing any code.

However just in case it is, this changes the eval to assign to a
static, so the eval will give a syntax error if an attacker
manages to make $value look like `ls` or some other expression