circular dependency when building LaTeX?

[Karl Berry]

    Interesting suggestion, thanks. Is trunk considered a stable version
    of the TeX Live distribution that can be relied upon by users?

The Master/ part of the TL source tree is the current runtime (TeX
files, etc.).  It can and should be considered the version to use, as
opposed to some arbitrarily old version. There is not really any such
thing as a "stable" version, because TeX is the joint effort of hundreds
of people working independently, uploading packages to CTAN.

Using the 2020.1 tag for your runtime is merely dooming your users to
having stale (and never-updated) versions of packages, not giving them
more stability. If anything, trunk is the most stable to use, because
new releases are often made to fix compatibility problems. This is more
true than ever this year, as the LaTeX core steadily makes changes
required for (ultimately) better accessibility, but which also require
changes on the part of many third-party packages.

The Build/ part of the TL source tree, on the other hand, is the
compiled sources. It is indeed better to use the 2020.1 tag of that
subtree for building your binaries, since that source tree does (as far
as we know) compile and work. The trunk version of the sources hopefully
compiles, but has usually barely been exercised. 

The tldistro at tug.org list (lists.tug.org/tldistro) is a list dedicated
to precisely the questions of adapting TL to a particular
system. Building formats, maps, etc., have all been discussed many times
(at least once per distro :). If questions persist, you might write
there, and/or check the archives there, etc. --best, karl.