.github: small improvements to the bug template

First, add a note about 'cue version' showing no version information if
one builds from source.

Second, remove the operating system question, as 'cue version' already
shows the equivalent of GOOS/GOARCH.

Change-Id: Id71516aaa529240b36a8b2e9491ac51c7095a607
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/4922
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
diff --git a/.github/ISSUE_TEMPLATE/bug_report.md b/.github/ISSUE_TEMPLATE/bug_report.md
index ee4f19f..781a070 100644
--- a/.github/ISSUE_TEMPLATE/bug_report.md
+++ b/.github/ISSUE_TEMPLATE/bug_report.md
@@ -19,13 +19,14 @@
 
 </pre>
 
+<!--
+If you built from source, specify what git tag or commit was used.
+-->
+
 ### Does this issue reproduce with the latest release?
 
 
 
-### What operating system are you using?
-
-
 ### What did you do?
 
 <!--