fix: support backup create when streaming to stdout

[ivanauth]

Summary

zed backup create - (write backup to stdout) currently fails with:

failed to store cursor in lockfile: cannot create temp file: open /dev/stdout.lock343325193: operation not permitted
sync /dev/stdout: inappropriate ioctl for device

OcfFileEncoder was attempting to write a cursor lockfile derived from fe.file.Name() (resolving to /dev/stdout.lock) and calling file.Sync() / file.Close() on os.Stdout during shutdown.

This was reported in the review of #688: #688 (review).

Closes #284.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 87.50000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 43.72%. Comparing base (97d4943) to head (c0d2b3b).

Files with missing lines	Patch %	Lines
pkg/backupformat/encoder.go	87.50%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #691      +/-   ##
==========================================
+ Coverage   43.60%   43.72%   +0.12%     
==========================================
  Files          38       38              
  Lines        6178     6193      +15     
==========================================
+ Hits         2694     2708      +14     
+ Misses       3204     3203       -1     
- Partials      280      282       +2     

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[miparnisari]

Do you mind updating the help text of zed backup create? Currently it says

$ ./zed backup create --help
Backup a permission system to a file

Usage:
  zed backup create <filename> [flags]

let's make it "Backup a permission system to a file or to stdout (denoted by a dash)"

and zed backup create <filename-or-dash>

[ivanauth]

Updated in ecc773a.