Unity's approved method for compiling the source of a Unity project into an executable is to install the Unity Editor - compiling either locally, or using it to communicate with a build server / cloud. That's the only method they offer or authorize.
If anyone were to provide a tool separate from the Unity Editor that could build Unity projects without a licensed Unity Editor installation, it would violate the Unity terms of service:
2.6 General Restrictions.
Except as expressly specified in this Agreement, you may not: (a) copy (except in the course of loading or
installing) or modify or create derivative works of the Unity
Software; (b) distribute, transfer, sublicense, lease, lend, rent or
otherwise provide access to the Unity Software to any third party; (c)
directly or indirectly make the functionality of the Unity Software
available to multiple users or third parties through any means,
including but not limited to by uploading the Unity Software to a
network or file-sharing service or through any hosting, application
services provider, service bureau, software-as-a-service (SaaS) or any
other technology or service; (d) use the Unity Software for
competitive analysis or to develop a competing product or service; or
(e) do anything that could cause or result in the Unity Software
(including the runtime portion thereof) being subject to any open
source license (or similar license) that requires as a condition of
use, modification or distribution that the Unity Software (including
the runtime portion thereof) or other software combined or distributed
with the Unity Software be: (i) disclosed or distributed in source
code form; (ii) licensed for the purpose of making derivative works;
or (iii) redistributable at no charge. You acknowledge and agree that
portions of the Unity Software, including but not limited to the
source code and the specific design and structure of individual
modules or programs, are confidential and constitute or contain trade
secrets of Unity and its licensors. Accordingly, you agree not to
disassemble, decompile, modify or reverse engineer the Unity Software,
in whole or in part, or permit or authorize a third party to do so,
except to the extent such activities are expressly permitted by this
Agreement or by law notwithstanding this prohibition. Notwithstanding
the restriction prohibiting decompiling in the immediately preceding
sentence, you may decompile the Unity Common Language Infrastructure
(CLI) assemblies solely to inspect their functionality for purposes of
understanding or improving performance of your Project Content or any
editor extension to the Unity Software.
