function keywordsEnvVar( keywords ) {
	return 'KEYWORDS=\'' + keywords.join( ',' ) + '\'';
}
Returns a `KEYWORDS` environment variable string.

@private
@param {StringArray} keywords - list of keywords
@returns {string} environment variable string

@example
var keywords = [ 'math', 'special' ];

var s = keywordsEnvVar( keywords );
// returns 'KEYWORDS=\'math,special\''