erlang/*: set github.tarball_from explicitly

In preparation for changing the default.

erlang/atomvm/Portfile

@@ -10,6 +10,8 @@ PortGroup               legacysupport 1.1
 legacysupport.newest_darwin_requires_legacy 16
 
 github.setup            atomvm atomvm 0.6.5 v
+# Change github.tarball_from to 'releases' or 'archive' next update
+github.tarball_from     tarball
 revision                0
 epoch                   1
 categories              erlang devel

erlang/erlsom/Portfile

@@ -5,6 +5,8 @@ PortGroup           github 1.0
 PortGroup           makefile 1.0
 
 github.setup        willemdj erlsom 1.5.1 v
+# Change github.tarball_from to 'releases' or 'archive' next update
+github.tarball_from tarball
 revision            0
 categories          erlang textproc
 license             LGPL-3+

erlang/rebar/Portfile

@@ -4,6 +4,8 @@ PortSystem      1.0
 PortGroup       github 1.0
 
 github.setup    rebar rebar 2.6.4
+# Change github.tarball_from to 'releases' or 'archive' next update
+github.tarball_from tarball
 categories      erlang devel
 platforms       any
 maintainers     {ciserlohn @ci42}