fix: panic when concurrency=0

[ldez]

When I created #4441, I tested the PR but I forgot that the default value is not 0 but the number of CPUs.

But when you set explicitly the default to 0, it panics:

$ go run ./cmd/golangci-lint run -j 0
c.cfg.Run.Concurrency 0
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x18 pc=0x1536a36]

goroutine 1 [running]:
go.uber.org/automaxprocs/maxprocs.Set({0xc000295c00, 0x1, 0xc000295c00?})
        /home/ldez/sources/go/pkg/mod/go.uber.org/automaxprocs@v1.5.3/maxprocs/maxprocs.go:89 +0x76
github.com/golangci/golangci-lint/pkg/commands.(*runCommand).persistentPreRunE(0xc0002a0ea0, 0xc00038c908, {0x93f76a?, 0xc0003c2600?, 0xc0003c2600?})
        /home/ldez/sources/go/src/github.com/golangci/golangci-lint/pkg/commands/run.go:164 +0x1b5
github.com/spf13/cobra.(*Command).execute(0xc00038c908, {0xc000f11a80, 0x2, 0x2})
        /home/ldez/sources/go/pkg/mod/github.com/spf13/cobra@v1.7.0/command.go:915 +0x764
github.com/spf13/cobra.(*Command).ExecuteC(0xc00038c308)
        /home/ldez/sources/go/pkg/mod/github.com/spf13/cobra@v1.7.0/command.go:1068 +0x3a5
github.com/spf13/cobra.(*Command).Execute(...)
        /home/ldez/sources/go/pkg/mod/github.com/spf13/cobra@v1.7.0/command.go:992
github.com/golangci/golangci-lint/pkg/commands.(*rootCommand).Execute(0xc0001f7590)
        /home/ldez/sources/go/src/github.com/golangci/golangci-lint/pkg/commands/root.go:83 +0x3d
github.com/golangci/golangci-lint/pkg/commands.Execute({{0x1a921f8, 0x8}, {0x1aeb0af, 0x7}, {0xc00030e330, 0x16}, {0x1817868, 0x9}})
        /home/ldez/sources/go/src/github.com/golangci/golangci-lint/pkg/commands/root.go:18 +0x5a
main.main()
        /home/ldez/sources/go/src/github.com/golangci/golangci-lint/cmd/golangci-lint/main.go:39 +0x1d1
exit status 2

I also updated the documentation based on the Go official doc about GOMAXPROCS: https://pkg.go.dev/runtime