reexec: add "CommandContext"

[thaJeztah]

• follow-up to / stacked on reexec: align closer to exec.Command #191

Adds a CommandContext command, to provide the equivalent of exec.CommandContext.

[thaJeztah]

Had to remove some bits from the test, because exec.Command.Cancel was added in go1.20, and we still test against go1.18 for this module;

go: no module dependencies to download
Error: reexec/reexec_test.go:172:8: cmd.Cancel undefined (type *exec.Cmd has no field or method Cancel) (typecheck)
make: *** [Makefile:19: foreach] Error 1
			cmd.Cancel = func() error {
			    ^

[thaJeztah]

Ah, derp, and of course, windows has a different error;

=== RUN   TestCommandContext/context_timeout
    reexec_test.go:190: got "exit status 1", want "signal: killed"

[thaJeztah]

• Moved the parts that required go1.20 to reexec: test cmd.Cancel, bump minimum go version to go1.20 #194

[Copilot]

Pull Request Overview

This PR introduces a CommandContext wrapper mirroring exec.CommandContext in the reexec package, enabling context-based cancellation for re-executed commands. Key changes:

• Added CommandContext API in reexec.go and platform-specific implementations in reexec_other.go and reexec_linux.go.
• Updated imports and registrations to support context-aware execution.
• Added TestCommandContext in reexec_test.go to validate context cancellation, timeouts, and argument handling.

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
reexec/reexec.go	Added public CommandContext function and imported context.
reexec/reexec_other.go	Introduced generic commandContext implementation.
reexec/reexec_linux.go	Added Linux-specific commandContext with Pdeathsig support.
reexec/reexec_test.go	New TestCommandContext covering basename, full path, args, cancel, and timeout cases.

Comments suppressed due to low confidence (2)

reexec/reexec_test.go:139

• Missing import of "path/filepath", which is required for filepath.Join. Please add "path/filepath" to the import block.

			cmdAndArgs: []string{filepath.Join("something", testReExec2)},

reexec/reexec_other.go:19

• This commandContext is duplicated in reexec_linux.go, leading to a redeclaration error on Linux builds. Add a build tag like // +build !linux at the top of this file to restrict it to non-Linux platforms.

func commandContext(ctx context.Context, args ...string) *exec.Cmd {