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/outie.sh b/build/outie.sh
new file mode 100755
index 0000000..f267167
--- /dev/null
+++ b/build/outie.sh
@@ -0,0 +1,5 @@
+#!/bin/bash
+set -e
+
+# Note: This incantation is duplicated in .goreleaser.yml; please keep them in sync.
+go build -ldflags="${LDFLAGS:-}" -tags=outie -o sketch ./cmd/sketch