main/yash: run add-shell in yash.post-upgrade

edit: most of below fixed by !65656 (merged)

fix

  / # apk add dash-binsh
  (1/3) Purging busybox-binsh (1.37.0-r8)
  (2/3) Installing dash (0.5.12-r2)
  Executing dash-0.5.12-r2.post-install
  ERROR: dash-0.5.12-r2.post-install: script exited with error 127
  (3/3) Installing dash-binsh (0.5.12-r2)
  Executing busybox-1.37.0-r8.trigger
  1 error; 14 MiB in 22 packages

and

  / # apk add yash-binsh
  (1/5) Purging busybox-binsh (1.37.0-r8)
  (2/5) Installing ncurses-terminfo-base (6.5_p20241006-r3)
  (3/5) Installing libncursesw (6.5_p20241006-r3)
  (4/5) Installing yash (2.57-r0)
  Executing yash-2.57-r0.post-install
  ERROR: yash-2.57-r0.post-install: script exited with error 127
  (5/5) Installing yash-binsh (2.57-r0)
  Executing busybox-1.37.0-r8.trigger
  1 error; 15 MiB in 24 package

dash - also use #!/usr/bin/dash in dash.pre-deinstall as it would still be available

yash - also use #!/usr/bin/yash in yash.post-upgrade and yash.pre-deinstall, as it would be available and may mitigate issues, like apk fix running yash.post-upgrade instead of yash.post-install