chore: Use sync.OnceValues to reduce redundant computation

[qianlongzt]

example

{{- if and (lookPath "mise") (eq .os "linux") }}
{{- output "mise" "completion" "fish" -}}
{{end -}}

time=2025-03-03T16:39:37.974+08:00 level=INFO msg=ReadFile component=system name=/home/qianlongzt/.local/share/chezmoi/home/private_dot_config/fish/completions/empty_mise.fish.tmpl size=101 data="{{- if and (lookPath \"mise\") (eq .os \"linux\") }}\n{{- output \"mis..."
time=2025-03-03T16:39:38.042+08:00 level=INFO msg=Output cmd="/home/linuxbrew/.linuxbrew/bin/mise completion fish" duration=66.693957ms size=696 output="# if \"usage\" is not installed show an error\nif ! command -v usag..." err=<nil>
time=2025-03-03T16:39:38.095+08:00 level=INFO msg=Output cmd="/home/linuxbrew/.linuxbrew/bin/mise completion fish" duration=52.165978ms size=696 output="# if \"usage\" is not installed show an error\nif ! command -v usag..." err=<nil>
time=2025-03-03T16:39:38.096+08:00 level=INFO msg=Lstat component=system name=/home/qianlongzt/.config/fish/completions/mise.fish

[twpayne]

Thank you very much for spotting this and for the excellent fix!

[twpayne]

Thank you again for this fix. I'd noticed that chezmoi had been slower than I expected for a while, but had not taken the time to investigate why. Your fix here gives a very clear and welcome performance boost. Thank you!