fix(use): re-use mise.toml if exists

[jdx]

Fixes #2205

[github-actions[bot]]

Package	Line Rate	Complexity	Health
src	87%	0	✔
src.cli	84%	0	✔
src.cli.alias	89%	0	✔
src.cli.args	96%	0	✔
src.cli.backends	95%	0	✔
src.cli.cache	95%	0	✔
src.cli.config	92%	0	✔
src.cli.direnv	80%	0	✔
src.cli.generate	87%	0	✔
src.cli.plugins	87%	0	✔
src.cli.settings	93%	0	✔
src.cli.sync	29%	0	❌
src.cli.tasks	71%	0	➖
src.config.config_file	89%	0	✔
src.config	87%	0	✔
src.forge	78%	0	✔
src.plugins.core	55%	0	➖
src.plugins	84%	0	✔
src.shell	98%	0	✔
src.toolset	86%	0	✔
src.ui	64%	0	➖
Summary	82% (13773 / 16835)	0	✔