Malformed informational log output for build context

[abg]

From a local build against v0.13.0:

$ make build
$ ./mysqld_exporter --version
mysqld_exporter, version 0.13.0 (branch: master, revision: ad2847c7fa67b9debafccd5a08bacb12fc9031f1)
  build user:       root@49837bc938f8
  build date:       20210526-20:12:58
  go version:       go1.16.3
  platform:         linux/amd64

Actual output:

$ ./mysqld_exporter level=info ts=2021-05-26T20:14:57.319Z caller=mysqld_exporter.go:277 msg="Starting msqyld_exporter" version="(version=0.13.0, branch=master, revision=ad2847c7fa67b9debafccd5a08bacb12fc9031f1)" level=info ts=2021-05-26T20:14:57.320Z caller=mysqld_exporter.go:278 msg="Build context" (gogo1.16.3,userroot@49837bc938f8,date20210526-20:12:58)=(MISSING)

Expected output:

$ ./mysqld_exporter ... level=info ts=2021-05-26T20:16:33.766Z caller=mysqld_exporter.go:278 msg="Build context" context="(go=go1.16.3, user=root@49837bc938f8, date=20210526-20:15:58)"