This code injects a script tag into the DOM, which means that the code executed will be in relation to the page itself, not the content script. So while you've probably successfully loaded jquery into the host page, the content script won't have access to the library since content scripts can't directly access javascript methods or objects in the host page.
The correct way to include jquery in a content script is shown here:
Namely, include a local copy of jQuery and then include that file in your content script declaration:
"content_scripts": [{
"matches": ["
http://www.foo.com/*"],
"js": ["jquery.js", "myscript.js"]
}],