fix(env): match Tanka's multiple-environments error format

[julienduchesne]

Summary

When an inline file resolves to multiple environments and no --name is given, rtk printed every match on a single comma-separated line. With dozens of inline environments that's painful to scan. Format the error like Tanka's ErrMultipleEnvs instead, listing one environment per line.

• Names are sorted and rendered one per line (\n - <name>), matching Tanka's output
• Reuse Tanka's wording, including the matching "<name>" variant when a --name filter still leaves multiple matches

Made with Cursor

[github-actions]

Benchmark Results

Benchmark	Test	vs tk	vs base
Diff	cluster_scoped	49.06x faster	equal
Diff	configmap_unchanged	29.38x faster	equal
Diff	deployment_nested_changes	28.31x faster	equal
Env List	Single Inline Directory (--json)	46.91x faster	equal
Env List	Single Inline File (--json)	46.8x faster	equal
Env List	All Environments (--json)	85.3x faster	equal
Eval	Single Static Environment	15.24x faster	equal
Eval	Inline Environment File	19.93x faster	equal
Eval	Eval Expression - Deeply Nested Path	15.88x faster	equal
Export (Full)	Single Static Environment	7.08x faster	equal
Export (Full)	Single Inline Environment	22.28x faster	equal
Export (Full)	All Environments	20.05x faster	equal
Export (Replace)	Single Static Environment	5.88x faster	equal
Export (Replace)	Single Inline Environment	21.43x faster	equal
Export (Replace)	All Environments	32.83x faster	equal
Helm Template	Full Export	1.42x faster	equal
Tool Importers Count	Single Lib Directory (Non-Recursive)	2.52x faster	equal
Tool Importers Count	Single Lib Directory (Recursive)	2.52x faster	equal
Tool Importers Count	Entire Lib Directory (Non-Recursive)	1.62x faster	equal
Tool Importers Count	Entire Lib Directory (Recursive)	1.13x faster	equal
Tool Importers	Global Lib File	3.17x faster	equal
Tool Importers	Single Env-Specific Lib File	1.68x faster	equal
Tool Importers	All Env-Specific Lib Files	5.17x faster	equal
Tool Importers	All Lib Files	5.07x faster	equal
Tool Importers	All Jsonnet Files	8.82x faster	equal
Tool Imports	Single Static Environment	3.31x faster	equal
Tool Imports	Single Inline Environment File	3.34x faster	equal
Validate Manifests	All Validations	-	equal
Validate Manifests	Manifest Tests Only	-	equal
Validate Manifests	Namespace Test Only	-	equal

Full results available in workflow artifacts.

Benchmark run on commit 9ea50df2da8d794816aa61899a73c693e3cd31c0