[PR PATCH] Manual.md: document `XBPS_BUILD_ENVIRONMENT`

[classabbyamp <classabbyamp@users.noreply.github.com>]

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There is a new pull request by classabbyamp against master on the void-packages repository

https://github.com/classabbyamp/void-packages manual/xbps-build-env
https://github.com/void-linux/void-packages/pull/39298

Manual.md: document `XBPS_BUILD_ENVIRONMENT`
Went looking for it and couldn't easily find it, so I documented it in the usual place


A patch file from https://github.com/void-linux/void-packages/pull/39298.patch is attached

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From 5bacf490c086af135074343c9f48e80832322da0 Mon Sep 17 00:00:00 2001
From: classabbyamp <void@placeviolette.net>
Date: Thu, 15 Sep 2022 06:01:21 -0400
Subject: [PATCH] Manual.md: document `XBPS_BUILD_ENVIRONMENT`

---
 Manual.md | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/Manual.md b/Manual.md
index c24be6e90a74..f0cb3ac3bac1 100644
--- a/Manual.md
+++ b/Manual.md
@@ -395,6 +395,8 @@ in this directory such as `${XBPS_BUILDDIR}/${wrksrc}`.
 
 - `XBPS_RUST_TARGET` The target architecture triplet used by `rustc` and `cargo`.
 
+- `XBPS_BUILD_ENVIRONMENT` Enables continuous-integration-specific operations. Set to `void-packages-ci` if in continuous integration.
+
 <a id="available_vars"></a>
 ### Available variables