fix: Race condition on execution when context closes

[kylecarbs]

Accessing ProcessState on non-linux operating systems immediately
after context cancels can conflict with the Wait() stdlib func:
https://github.com/golang/go/blob/36b81acfa19d9fedf6a0cd60c394fd7a7703834e/src/os/exec/exec.go#L511

[kylecarbs]

==================
WARNING: DATA RACE
Write at 0x00c000bf5548 by goroutine 33:
  os/exec.(*Cmd).Wait()
      /opt/hostedtoolcache/go/1.17.6/x64/src/os/exec/exec.go:511 +0x15d
  os/exec.(*Cmd).Run()
      /opt/hostedtoolcache/go/1.17.6/x64/src/os/exec/exec.go:341 +0x64
  github.com/hashicorp/terraform-exec/tfexec.(*Terraform).runTerraformCmd()
      /home/runner/go/pkg/mod/github.com/kylecarbs/[email protected]/tfexec/cmd_linux.go:45 +0x4a7
  github.com/hashicorp/terraform-exec/tfexec.(*Terraform).version()
      /home/runner/go/pkg/mod/github.com/kylecarbs/[email protected]/tfexec/version.go:55 +0x193
  github.com/hashicorp/terraform-exec/tfexec.(*Terraform).Version()
      /home/runner/go/pkg/mod/github.com/kylecarbs/[email protected]/tfexec/version.go:39 +0x12d
  github.com/coder/coder/provisioner/terraform.(*terraform).Provision()
      /home/runner/work/coder/coder/provisioner/terraform/provision.go:33 +0x2e4
  github.com/coder/coder/provisionersdk/proto.DRPCProvisionerDescription.Method.func2()
      /home/runner/work/coder/coder/provisionersdk/proto/provisioner_drpc.pb.go:161 +0x108
  storj.io/drpc/drpcmux.(*Mux).HandleRPC()
      /home/runner/go/pkg/mod/storj.io/[email protected]/drpcmux/handle_rpc.go:33 +0x305
  storj.io/drpc/drpcserver.(*Server).handleRPC()
      /home/runner/go/pkg/mod/storj.io/[email protected]/drpcserver/server.go:122 +0x71
  storj.io/drpc/drpcserver.(*Server).ServeOne()
      /home/runner/go/pkg/mod/storj.io/[email protected]/drpcserver/server.go:66 +0x2da
  storj.io/drpc/drpcserver.(*Server).Serve.func2()
      /home/runner/go/pkg/mod/storj.io/[email protected]/drpcserver/server.go:112 +0x76
  storj.io/drpc/drpcctx.(*Tracker).track()
      /home/runner/go/pkg/mod/storj.io/[email protected]/drpcctx/transport.go:52 +0x3d
  storj.io/drpc/drpcctx.(*Tracker).Run·dwrap·1()
      /home/runner/go/pkg/mod/storj.io/[email protected]/drpcctx/transport.go:47 +0x47

Previous read at 0x00c000bf5548 by goroutine 119:
  github.com/hashicorp/terraform-exec/tfexec.(*Terraform).runTerraformCmd.func1()
      /home/runner/go/pkg/mod/github.com/kylecarbs/[email protected]/tfexec/cmd_linux.go:26 +0x185

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