docs: document PMB_APK_FORCE_MISSING_REPOSITORIES (MR 2506)

docs/environment_variables.md

@@ -5,3 +5,11 @@
 pmbootstrap supports `doas` and `sudo`. If multiple sudo implementations are
 installed, pmbootstrap will use `doas`. You can set the `PMB_SUDO`
 environmental variable to define the sudo implementation you want to use.
+
+## `PMB_APK_FORCE_MISSING_REPOSITORIES`
+
+When this is set to `1`, pmbootstrap will not complain if downloading an
+APKINDEX results in a 404 not found error. This is used by
+[bpo](https://build.postmarketos.org) when building a new stable repository for
+the first time. For example if the `x86_64` repository was already built and
+published, but the `aarch64` repository wasn't published yet.