feat: Deno.execPath() no longer requires --allow-read permission

[bartlomieju]

This commit changes Deno.execPath() API to no longer require
read permission.

This change is dictated by the fact that in common scenarios, requiring
read permission is less secure than not requiring permissions - if
a user wants to spawn a Deno subprocess using the current executable,
they would do something like:

new Deno.Command(Deno.execPath(), { args: ["eval", "1+1"] }).outputSync();

To run this program, currently one needs to pass --allow-read --allow-run=deno flags.
It's possible to limit scope of --allow-read flag, but it's really cumbersome to do,
so most users will opt to give a blanket --allow-read permission.

Not requiring read permissions allows the above program to be run with
just --allow-run=deno flag.

This change is in similar to relaxing of permissions in Deno.cwd() API done
in #27192.

Ref #20061 (comment)