fix: set LANG=C for locale-dependent tests

[dolmen]

Set LANG=C for tests which call external commands (bash, git) which have locale-dependent output.

Fixes: #2196

$ LANG=fr_FR.UTF-8 go test -trimpath -count=1 ./...
?   	github.com/asdf-vm/asdf/internal/cli	[no test files]
?   	github.com/asdf-vm/asdf/internal/installtest	[no test files]
?   	github.com/asdf-vm/asdf/internal/repotest	[no test files]
ok  	github.com/asdf-vm/asdf/cmd/asdf	102.088s
ok  	github.com/asdf-vm/asdf/internal/cli/set	0.181s
ok  	github.com/asdf-vm/asdf/internal/completions	1.065s
ok  	github.com/asdf-vm/asdf/internal/config	0.697s
ok  	github.com/asdf-vm/asdf/internal/data	0.357s
ok  	github.com/asdf-vm/asdf/internal/exec	1.584s
ok  	github.com/asdf-vm/asdf/internal/execenv	2.300s
ok  	github.com/asdf-vm/asdf/internal/execute	1.347s
ok  	github.com/asdf-vm/asdf/internal/git	3.107s
ok  	github.com/asdf-vm/asdf/internal/help	2.933s
ok  	github.com/asdf-vm/asdf/internal/hook	1.071s
ok  	github.com/asdf-vm/asdf/internal/info	0.969s
ok  	github.com/asdf-vm/asdf/internal/installs	2.459s
ok  	github.com/asdf-vm/asdf/internal/paths	0.668s
ok  	github.com/asdf-vm/asdf/internal/pluginindex	1.260s
ok  	github.com/asdf-vm/asdf/internal/plugins	9.831s
ok  	github.com/asdf-vm/asdf/internal/resolve	3.537s
ok  	github.com/asdf-vm/asdf/internal/shims	10.938s
ok  	github.com/asdf-vm/asdf/internal/toolversions	0.825s
ok  	github.com/asdf-vm/asdf/internal/versions	18.840s

[Stratus3D]

Thanks for the issue report and the PR to fix it @dolmen !