Ensure URL encoding is supported in SYSTEM_CAPABILITIES

[bhearsum]

We currently have values such as ISET:SSE4_2,MEM:16192 in this field, which are technically invalid in an URL. https://bugzilla.mozilla.org/show_bug.cgi?id=1620327 is seeking to URL encode these to make them vaild. We need to make sure this is supported before that lands.