Use the "sh" package where applicable

[kke]

There were several places there rig didn't "dogfood" on the sh package for building commands.

[Copilot]

Pull request overview

This PR updates several remote command invocations to “dogfood” the internal sh package for safer/cleaner command construction instead of manual fmt.Sprintf(...) + shell-escaping.

Changes:

• Replaced several fmt.Sprintf(...)-built commands in PosixFS with sh.Command(...) (e.g., chmod, install, mkdir, printenv).
• Reworked PosixFile dd calls to use sh.Command(...) rather than formatting a single shell string.
• Updated Upstart “disable service” to write the override file using tee with stdin instead of shell redirection.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
remotefs/posixfs.go	Migrates multiple filesystem operations to sh.Command(...) and changes Getenv to printenv.
remotefs/posixfile.go	Migrates dd-based read/write/copy operations to sh.Command(...).
initsystem/upstart.go	Removes manual shell escaping and writes override files via tee + stdin.

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[Copilot]

Pull request overview

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

[Copilot]

Pull request overview

Copilot reviewed 7 out of 7 changed files in this pull request and generated no new comments.