[go] Add "$FUCHSIA_DIR" to fuchsia.txt regen command

This is needed to regen from a directory other than
$FUCHSIA_DIR, for instance third_party/go.

Change-Id: I9a64e7a0228ab8c45937059d4f0003b3813183b5
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/go/+/430726
Reviewed-by: Pascal Perez <pascallouis@google.com>
Commit-Queue: Benjamin Prosnitz <bprosnitz@google.com>
diff --git a/api/README b/api/README
index 846e31e..d8746d3 100644
--- a/api/README
+++ b/api/README
@@ -15,4 +15,4 @@
 fuchsia.txt lists Fuchsia specific features. After a full build, this
 list can be regenerated with
 
-    $(fx get-build-dir)/host-tools/goroot/bin/go tool api | grep -e '^pkg syscall/zx' > third_party/go/api/fuchsia.txt
+    $(fx get-build-dir)/host-tools/goroot/bin/go tool api | grep -e '^pkg syscall/zx' > ${FUCHSIA_DIR}/third_party/go/api/fuchsia.txt