build: perform one-by-one build

[rockdrilla]

What this PR does / why we need it:

1. avoid duplicate package requests to ${GOPROXY} and ${GOSUMDB} - first binary target fetches all required packages and further binary builds reuse them.

2. respect ${GOMAXPROCS} (if any).

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[xnyo]

Hi and thank you for contributing to Grafana!

I have some comments about your change:

[xnyo]

I would keep the parallel execution to leverage all cores when building plugins, however your point about duplicate package requests is a fair one.

To address it, I would change it so fetching the packages happens as a separate step before building the executable, which is then done in parallel, like so:

// GoModDownload downloads the packages required by the executable by running "go mod download".
func (Build) GoModDownload() error {
	return sh.Run("go", "mod", "download")
}

// BuildAll builds production executables for all supported platforms.
func BuildAll() { //revive:disable-line
	b := Build{}
	mg.Deps(b.GoModDownload)
	mg.Deps(b.Linux, b.Windows, b.Darwin, b.DarwinARM64, b.LinuxARM64, b.LinuxARM, b.GenerateManifestFile)
}

[rockdrilla]

Hi again!

Each binary target does go mod download under hood (if necessary) so separate download target isn't needed.

Furthermore, each binary target utilizes as much cores as possible (or at most ${GOMAXPROCS} if specified). Running all of them at once will throttle overall system/container.