The race detector was picking up a data race because of the
unsynchronized reassignment of ctx:
```
$ go run -race ./cmd/influxd
...
^C
==================
WARNING: DATA RACE
Write at 0x00c00053d220 by main goroutine:
main.platformF()
/Users/mr/gomod/platform/cmd/influxd/main.go:381 +0x2cf7
github.com/spf13/cobra.(*Command).execute()
/Users/mr/go/pkg/mod/github.com/spf13/cobra@v0.0.3/command.go:766 +0x8b2
github.com/spf13/cobra.(*Command).ExecuteC()
/Users/mr/go/pkg/mod/github.com/spf13/cobra@v0.0.3/command.go:852 +0x432
github.com/spf13/cobra.(*Command).Execute()
/Users/mr/go/pkg/mod/github.com/spf13/cobra@v0.0.3/command.go:800 +0x38
main.Execute()
/Users/mr/gomod/platform/cmd/influxd/main.go:388 +0x4e
main.main()
/Users/mr/gomod/platform/cmd/influxd/main.go:46 +0x2f
Previous read at 0x00c00053d220 by goroutine 147:
main.platformF.func1()
/Users/mr/gomod/platform/cmd/influxd/main.go:330 +0x3c
Goroutine 147 (running) created at:
main.platformF()
/Users/mr/gomod/platform/cmd/influxd/main.go:329 +0x2554
github.com/spf13/cobra.(*Command).execute()
/Users/mr/go/pkg/mod/github.com/spf13/cobra@v0.0.3/command.go:766 +0x8b2
github.com/spf13/cobra.(*Command).ExecuteC()
/Users/mr/go/pkg/mod/github.com/spf13/cobra@v0.0.3/command.go:852 +0x432
github.com/spf13/cobra.(*Command).Execute()
/Users/mr/go/pkg/mod/github.com/spf13/cobra@v0.0.3/command.go:800 +0x38
main.Execute()
/Users/mr/gomod/platform/cmd/influxd/main.go:388 +0x4e
main.main()
/Users/mr/gomod/platform/cmd/influxd/main.go:46 +0x2f
==================
```
So just assign context.WithCancel to a new variable instead.