command: apply SliceFlagSeparator to env-var values in PostParse

[alliasgher]

Summary

SliceFlagSeparator (and DisableSliceFlagSeparator) were not applied to flag values sourced from environment variables.

setMultiValueParsingConfig — which propagates the command's separator settings to the flag's underlying value — was only called inside cmd.set(). That path is taken when parsing CLI flags. When values come from environment variables they flow through flag.PostParse(), which called f.Set() without first configuring the separator. As a result the flag's value always used the default "," separator regardless of what SliceFlagSeparator was set to.

Before

export A="a;b;c"
./myapp  # SliceFlagSeparator: ";"
# cmd.StringSlice("a") → ["a;b;c"]  ← NOT split

After

export A="a;b;c"
./myapp  # SliceFlagSeparator: ";"
# cmd.StringSlice("a") → ["a", "b", "c"]  ← correctly split

Fix

Add a cmd.setMultiValueParsingConfig(flag) call in the PostParse loop (in command_run.go) before flag.PostParse() so the separator is configured regardless of whether the value came from the CLI or an env var.

Fixes #2262

Checklist

• Test added (TestCommandSliceFlagSeparatorFromEnvVar)
• Full test suite passes