Fix SIQGUIT signal handling

We started seeing this crash with our buildkite agents on Linux:

```
SIGQUIT: quit
PC=0x468c21 m=0 sigcode=0

goroutine 0 [idle]:
runtime.futex(0x1b35a28, 0x80, 0x0, 0x0, 0x0, 0x7fffe24203e8, 0x43a7b3, 0xc000280148, 0x7fffe2420408, 0x40ae9f, ...)
/usr/local/go/src/runtime/sys_linux_amd64.s:567 +0x21
runtime.futexsleep(0x1b35a28, 0x7fff00000000, 0xffffffffffffffff)
/usr/local/go/src/runtime/os_linux.go:44 +0x46
runtime.notesleep(0x1b35a28)
/usr/local/go/src/runtime/lock_futex.go:151 +0x9f
runtime.stoplockedm()
/usr/local/go/src/runtime/proc.go:1971 +0x88
runtime.schedule()
/usr/local/go/src/runtime/proc.go:2454 +0x4a6
runtime.park_m(0xc000082900)
/usr/local/go/src/runtime/proc.go:2690 +0x9d
runtime.mcall(0x0)
/usr/local/go/src/runtime/asm_amd64.s:318 +0x5b

goroutine 1 [syscall]:
syscall.Syscall6(0xf7, 0x1, 0x63ee, 0xc00021e920, 0x1000004, 0x0, 0x0, 0xc00021e9a0, 0x0, 0xc00021ea00)
/usr/local/go/src/syscall/asm_linux_amd64.s:41 +0x5
os.(*Process).blockUntilWaitable(0xc000383bf0, 0xc0000b0780, 0x3, 0xc00021ea00)
/usr/local/go/src/os/wait_waitid.go:31 +0x98
os.(*Process).wait(0xc000383bf0, 0x2c, 0xc000383c20, 0xc000380000)
/usr/local/go/src/os/exec_unix.go:22 +0x39
os.(*Process).Wait(...)
/usr/local/go/src/os/exec.go:125
os/exec.(*Cmd).Wait(0xc0003b22c0, 0x120da40, 0x29)
/usr/local/go/src/os/exec/exec.go:502 +0x60
github.com/buildkite/agent/v3/process.(*Process).Run(0xc0003b01c0, 0x0, 0x0)
/work/process/process.go:233 +0x472
github.com/buildkite/agent/v3/bootstrap/shell.(*Shell).executeCommand(0xc0002aa000, 0xc0003ae280, 0x13adfa0, 0xc0000ba008, 0x10101, 0x0, 0x0)
/work/bootstrap/shell/shell.go:403 +0x29b
github.com/buildkite/agent/v3/bootstrap/shell.(*Shell).RunWithoutPrompt(0xc0002aa000, 0xc0003944d0, 0x9, 0xc0003be490, 0x3, 0x5, 0x3, 0x6)
/work/bootstrap/shell/shell.go:209 +0x186
github.com/buildkite/agent/v3/bootstrap.(*Bootstrap).defaultCommandPhase(0xc0001e3900, 0x0, 0x0)
/work/bootstrap/bootstrap.go:1481 +0x8b9
github.com/buildkite/agent/v3/bootstrap.(*Bootstrap).CommandPhase(0xc0001e3900, 0x7, 0x1)
/work/bootstrap/bootstrap.go:1310 +0x67d
github.com/buildkite/agent/v3/bootstrap.(*Bootstrap).Run(0xc0001e3900, 0x13c1960, 0xc0000bfb00, 0x0)
/work/bootstrap/bootstrap.go:144 +0x4aa
github.com/buildkite/agent/v3/clicommand.glob..func6(0xc0000e1080)
/work/clicommand/bootstrap.go:418 +0x97c
github.com/urfave/cli.HandleAction(0xf28260, 0x122bdb8, 0xc0000e1080, 0x0, 0xc0000bd440)
/go/pkg/mod/github.com/urfave/cli@v0.0.0-20180226030253-8e01ec4cd3e2/app.go:503 +0x7c
github.com/urfave/cli.Command.Run(0x11e8944, 0x9, 0x0, 0x0, 0x0, 0x0, 0x0, 0x11fcbae, 0x1b, 0x0, ...)
/go/pkg/mod/github.com/urfave/cli@v0.0.0-20180226030253-8e01ec4cd3e2/command.go:165 +0x4da
github.com/urfave/cli.(*App).Run(0xc0001c61c0, 0xc0000ac020, 0x2, 0x2, 0x0, 0x0)
/go/pkg/mod/github.com/urfave/cli@v0.0.0-20180226030253-8e01ec4cd3e2/app.go:259 +0x775
main.main()
/work/main.go:115 +0x55d

goroutine 18 [syscall]:
os/signal.signal_recv(0x13ba360)
/usr/local/go/src/runtime/sigqueue.go:147 +0x9c
os/signal.loop()
/usr/local/go/src/os/signal/signal_unix.go:23 +0x22
created by os/signal.Notify.func1
/usr/local/go/src/os/signal/signal.go:127 +0x44

goroutine 56 [syscall]:
syscall.Syscall(0x0, 0x3, 0xc000272000, 0x8000, 0x4cb2f5, 0x7f52ff9beee8, 0x72)
/usr/local/go/src/syscall/asm_linux_amd64.s:18 +0x5
syscall.read(0x3, 0xc000272000, 0x8000, 0x8000, 0x0, 0x0, 0x0)
/usr/local/go/src/syscall/zsyscall_linux_amd64.go:686 +0x5a
syscall.Read(...)
/usr/local/go/src/syscall/syscall_unix.go:189
internal/poll.(*FD).Read(0xc0003be4e0, 0xc000272000, 0x8000, 0x8000, 0x0, 0x0, 0x0)
/usr/local/go/src/internal/poll/fd_unix.go:165 +0x133
os.(*File).read(...)
/usr/local/go/src/os/file_unix.go:263
os.(*File).Read(0xc0003bc080, 0xc000272000, 0x8000, 0x8000, 0x57, 0x0, 0x0)
/usr/local/go/src/os/file.go:116 +0x71
io.copyBuffer(0x13adfa0, 0xc0000ba008, 0x13adf80, 0xc0003bc080, 0xc000272000, 0x8000, 0x8000, 0x1372428, 0x1, 0x43d526)
/usr/local/go/src/io/io.go:405 +0x122
io.Copy(...)
/usr/local/go/src/io/io.go:364
github.com/buildkite/agent/v3/process.(*Process).Run.func2(0xc0003b01c0, 0xc0003bc080, 0xc0003867c0, 0xc000394590)
/work/process/process.go:178 +0xa9
created by github.com/buildkite/agent/v3/process.(*Process).Run
/work/process/process.go:173 +0x39d

goroutine 57 [select]:
github.com/buildkite/agent/v3/process.(*Process).Run.func3(0xc0003b01c0)
/work/process/process.go:214 +0xd8
created by github.com/buildkite/agent/v3/process.(*Process).Run
/work/process/process.go:213 +0x9a1

rax 0xca
rbx 0x1b358e0
rcx 0x468c23
rdx 0x0
rdi 0x1b35a28
rsi 0x80
rbp 0x7fffe24203d0
rsp 0x7fffe2420388
r8 0x0
r9 0x0
r10 0x0
r11 0x286
r12 0x0
r13 0x1
r14 0xc000094c00
r15 0x0
rip 0x468c21
rflags 0x286
cs 0x33
fs 0x0
gs 0x0
```

This is intermittent with our agents running on k8s, let me know what other info I can provide to debug this.

Start with process/process.go, especially Process.Run, and trace how bootstrap/shell/shell.go invokes it while handling signals on Linux. Reproduce the intermittent SIGQUIT failure with a Kubernetes-hosted agent, then verify that agent command execution no longer crashes and that relevant process behavior remains intact.

Written by the indexing model from the issue text.