perf: optimize shell object cloning

[reubeno]

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[github-actions]

Test Results

    2 files      9 suites   1m 15s ⏱️
  569 tests   569 ✅ 0 💤 0 ❌
1 124 runs  1 124 ✅ 0 💤 0 ❌

Results for commit 7124ca6.

[github-actions]

Performance Benchmark Report

Benchmark name	Baseline (μs)	Test/PR (μs)	Delta (μs)	Delta %
expand_one_string	3.52 μs	3.55 μs	0.03 μs	⚪ Unchanged
for_loop	578821.98 μs	609004.24 μs	30182.27 μs	⚪ Unchanged
instantiate_shell	61.77 μs	79.75 μs	17.97 μs	🟠 +29.09%
instantiate_shell_with_init_scripts	28508.42 μs	28775.92 μs	267.49 μs	🟠 +0.94%
parse_bash_completion	2846.78 μs	2872.86 μs	26.08 μs	⚪ Unchanged
parse_sample_script	4.27 μs	4.47 μs	0.20 μs	🟠 +4.73%
run_echo_builtin_command	90.97 μs	113.58 μs	22.61 μs	🟠 +24.86%
run_one_builtin_command	110.73 μs	131.84 μs	21.11 μs	🟠 +19.06%
run_one_external_command	1796.52 μs	1803.21 μs	6.69 μs	⚪ Unchanged
run_one_external_command_directly	819.13 μs	818.48 μs	-0.66 μs	⚪ Unchanged

Code Coverage Report: Only Changed Files listed

Package	Base Coverage	New Coverage	Difference
brush-core/src/builtins/set.rs	🟠 60.69%	🟠 61.36%	🟢 0.67%
brush-core/src/commands.rs	🟢 86.83%	🟢 86.68%	🔴 -0.15%
brush-core/src/interp.rs	🟢 92.35%	🟢 92.38%	🟢 0.03%
brush-core/src/openfiles.rs	🔴 47.53%	🔴 45.07%	🔴 -2.46%
brush-core/src/shell.rs	🟢 82.42%	🟢 82.29%	🔴 -0.13%
Overall Coverage	🟢 77.34%	🟢 77.35%	🟢 0.01%

Minimum allowed coverage is 70%, this run produced 77.35%