docs: add missing default stamp var (#865)

* docs: add missing default stamp var

* update docs

docs/stamping.md

diff --git a/docs/stamping.md b/docs/stamping.md
index 972be9c..171905c 100644
--- a/docs/stamping.md
+++ b/docs/stamping.md
@@ -16,7 +16,8 @@
 This means that a changed status variable only causes that action, not re-compilation and thus does not cause cascading re-builds.
 
 Bazel provides a couple of statuses by default, such as `BUILD_EMBED_LABEL` which is the value of the `--embed_label`
-argument, as well as `BUILD_HOST` and `BUILD_USER`. You can supply more with the workspace status script, see below.
+argument, as well as `BUILD_HOST`, `BUILD_TIMESTAMP`, and `BUILD_USER`.
+You can supply more with the workspace status script, see below.
 
 Some rules accept an attribute that uses the status variables.
 They will usually say something like "subject to stamp variable replacements".

lib/stamping.bzl

diff --git a/lib/stamping.bzl b/lib/stamping.bzl
index b54977d..4e1ce05 100644
--- a/lib/stamping.bzl
+++ b/lib/stamping.bzl
@@ -14,7 +14,8 @@
 This means that a changed status variable only causes that action, not re-compilation and thus does not cause cascading re-builds.
 
 Bazel provides a couple of statuses by default, such as `BUILD_EMBED_LABEL` which is the value of the `--embed_label`
-argument, as well as `BUILD_HOST` and `BUILD_USER`. You can supply more with the workspace status script, see below.
+argument, as well as `BUILD_HOST`, `BUILD_TIMESTAMP`, and `BUILD_USER`.
+You can supply more with the workspace status script, see below.
 
 Some rules accept an attribute that uses the status variables.
 They will usually say something like "subject to stamp variable replacements".