alloc exec leaks a goroutine on exit

[tgross]

On Nomad 1.9.0, upon exiting an alloc exec, we're leaking a goroutine:

goroutine 1417 [select]:
io.(*pipe).read(0xc0011ff980, {0xc001956000, 0x8000, 0x473201?})
    io/pipe.go:57 +0xa5
io.(*PipeReader).Read(0x4aa0840?, {0xc001956000?, 0xc001c4b750?, 0x14d92f5?})
    io/pipe.go:134 +0x1a
io.copyBuffer({0x3387400, 0xc0014e45e8}, {0x3383d80, 0xc0011ff980}, {0x0, 0x0, 0x0})
    io/io.go:429 +0x191
io.Copy(...)
    io/io.go:388
github.com/hashicorp/nomad/drivers/docker.(*Driver).ExecTaskStreaming.func3()
    github.com/hashicorp/nomad/drivers/docker/driver.go:1850 +0x7a
created by github.com/hashicorp/nomad/drivers/docker.(*Driver).ExecTaskStreaming in goroutine 1407
    github.com/hashicorp/nomad/drivers/docker/driver.go:1849 +0x69f

This looks superficially similar to what we're fixing in #24202 but appear to be a separate issue.