test: change formatting of fixtures/keys/Makefile

Converts the whitespace to spaces in the all: ... target for
consistency. The other whitespace has to remain tabs due to how
Makefiles work.

PR-URL: https://github.com/nodejs/node/pull/27962


Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Ujjwal Sharma <usharma1998@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>