cmd/snap: tweak the output of snap debug timings --ensure=...

[stolowski]

Tweak the output of ensure timings per the comment from aready landed PR - #6793 (comment)

With this change the ensure timings will have an artifical line with ensure tag printed first, followed by the ensure timings "nested" under it, e.g.

$ ./snap debug timings --ensure=auto-refresh --all
ID            Status        Doing      Undoing  Summary
auto-refresh                    -            -  
 ^                          604ms            -    query store and setup auto-refresh change
481           Done            5ms            -  Ensure prerequisites for "core18" are available
482           Done         1454ms            -  Download snap "core18" (970) from channel "stable"
 ^                         1442ms            -    download snap "core18"
483           Done          305ms            -  Fetch and check assertions for snap "core18" (970)
484           Done          537ms            -  Mount snap "core18" (970)
 ^                          211ms            -    check snap "core18"
 ^                          286ms            -    setup snap "core18"
485           Done           10ms            -  Run pre-refresh hook of "core18" snap if present
486           Done           13ms            -  Stop snap "core18" services
487           Done           48ms            -  Remove aliases for snap "core18"
488           Done           24ms            -  Make current revision for snap "core18" unavailable
489           Done           92ms            -  Copy snap "core18" data
490           Done           10ms            -  Setup snap "core18" (970) security profiles
491           Done           75ms            -  Make snap "core18" (970) available to the system
 ^                           42ms            -    update font config caches
492           Done           17ms            -  Automatically connect eligible plugs and slots of snap "core18"
493           Done           12ms            -  Set automatic aliases for snap "core18"
494           Done           18ms            -  Setup snap "core18" aliases
495           Done           17ms            -  Run post-refresh hook of "core18" snap if present
496           Done            9ms            -  Start snap "core18" (970) services
497           Done           28ms            -  Remove data for snap "core18" (782)
498           Done          564ms            -  Remove snap "core18" (782) from the system
499           Done            0ms            -  Clean up "core18" (970) install
500           Done          397ms            -  Consider re-refresh of "core18"

[bboozzoo]

Tried it out locally. LGTM.