The answer is: make a custom script!
eg: subl /home/user/buildBibtex.sh
consisting of:
#!/bin/sh
bibtex *.aux
run chmod +x /home/user/buildBibtex.sh
and the build file will look like this:
{
"cmd": ["/home/user/buildBibtex.sh"],
"selector": "text.bibtex"
}