Add option to skip upgrade check via env variable

[mszostok]

Description

Add option to skip upgrade check via env variable. It should be done via option, so such thing:

// when set environment HELMFILE_UPGRADE_NOTICE_DISABLED any value, skip upgrade notice.
var versionOpts []extension.CobraOption
if os.Getenv("HELMFILE_UPGRADE_NOTICE_DISABLED") == "" {
	versionOpts = append(versionOpts, extension.WithUpgradeNotice("helmfile", "helmfile"))
}

// ...
extension.NewVersionCobraCmd(
	versionOpts...,
),

can be optimized to:

extension.NewVersionCobraCmd(
  extension.WithUpgradeNotice("helmfile", "helmfile",
    upgrade.WithSkipOnEnv("HELMFILE_UPGRADE_NOTICE_DISABLED"),
  ),
)

By default, setting below envs to any value should also disable the update check:

• {CLI_NAME}_NO_UPDATE_NOTIFIER - matches the gh CLI style
• {CLI_NAME}_UPGRADE_NOTICE_DISABLED - matches the helmfile style

See the helmfile usage: helmfile/helmfile#412

Reasons

Enhance UX