Update build.md

[AnanyaKumar]

Make it clear how to run cadvisor after building

[googlebot]

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[cadvisorJenkinsBot]

Can one of the admins verify this patch?

[AnanyaKumar]

I signed the CLA!

[googlebot]

CLAs look good, thanks!

[vmarmol]

LGTM, thanks @AnanyaKumar!