fix(fish): honor GO_TASK_PROGNAME for experiments cache

[SergioChan]

Summary

• use $GO_TASK_PROGNAME instead of hardcoded task when refreshing fish completion experiment cache
• fixes fish completion warnings on distros where the binary name is go-task
• keeps completion behavior unchanged for default task installs

Testing

• git diff --check
• go test ./... (not runnable in this environment: required Go toolchain go1.25 is unavailable via GOTOOLCHAIN download)

Related

• Fixes Task completion with the fish shell raise a warning when distro use the go-task name instead of task #2727