how best to set the `projectile-regenerate-tags` command

Question

I have a fortran project I'm working on and I cannot figure out the right way to use .dir-locals to set the projectile-tags-command

here is my dir-locals.el file:

(
 (f90-mode
  (projectile-tags-command . "make -C ~/path/to/src etags")
  )
 (projectile-global-mode
  (projectile-tags-command . "make -C ~/path/to/src etags")
  )
 (projectile-mode
  (projectile-tags-command . "make -C ~/path/to/src etags")
  )
 )

as far as I can tell, the f90-mode entry is the only one that has an impact. If I don't have that there, the others do not touch the projectile-command variable when I am editing a fortran file, and I can't get anything to work when I'm in anything other than a fortran file buffer.

I thought that the projectile-mode entry should do the trick, but I don't understand why it isn't and I'm wondering how I can achieve this, namely that whenever I'm in a buffer relevant to that .dir-locals I set the tags command for projectile