Substitute command keys in 'ielm-header' at use time

Substitute command keys in 'ielm-header' at use time

Before, command keys were substituted into the ielm-header when
ielm.el was loaded, which resulted in the substitutions depending on
the user's current buffer instead of the ielm buffer.
For example, if the user was in an info-mode buffer, the key would
appear as 'H' instead of 'C-h m'.
Now, the command key is substituted after the ielm buffer has been
created.
* lisp/ielm.el (ielm-header): Remove substitute-command-keys.
(inferior-emacs-lisp-mode): Add substitute-command-keys.  (Bug#65213)

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