gdb/doc: use @env to reference environment variables

Clean up a few places where we are not using @env{...} to reference
environment variables.

gdb/doc/ChangeLog:

	* gdb.texinfo (Initialization Files): Use @env when referencing
	environment variables.
	(Shell Commands): Likewise.
	(Starting): Likewise.
	(Arguments): Likewise.
	(Environment): Likewise.
	(Edit): Likewise.
	(Compiling and Injecting Code): Likewise.
	(Files): Likewise.
	(Command History): Likewise.
	(Screen Size): Likewise.
	(Emacs): Likewise.