all: re-organize build/release

- extract steps into shell files
- fix GoReleaser builds (I hope)
- rely more on the toolchain for version information
- allow non-standard builds, but print a warning

Suggested-by: Marc-Antoine Ruel <maruel@gmail.com>
diff --git a/build/webui.sh b/build/webui.sh
new file mode 100755
index 0000000..0dfc2ed
--- /dev/null
+++ b/build/webui.sh
@@ -0,0 +1,5 @@
+#!/bin/bash
+set -e
+
+rm -rf embedded/webui-dist
+unset GOOS GOARCH && go run ./cmd/genwebui -- embedded/webui-dist