Run the tests with cgo off and without the race detector (closes #538)

sneak ruled the race detector out
(#538 (comment)), and on
Linux it was the only reason the tests needed cgo. The test phase and the
browser test binary now build with CGO_ENABLED=0 and without -race, so no
test binary compiles mattn's C SQLite. The test phase keeps the builder
stage's Debian Go image. Its comments, the README and the code comments that
said the suite runs under -race now say what is true, and the README notes
the exception to the vendored REPO_POLICIES.md.

Model: opus-5-5

internal/server/early_shutdown_test.go

@@ -10,14 +10,12 @@ import (
 	"sneak.berlin/go/webhooker/internal/server"
 )
 
-// earlyStopIterations is how many start/stop cycles the race test
+// earlyStopIterations is how many start/stop cycles the test below
 // runs. The window it aims at is the gap between the OnStart hook
 // returning and the serving goroutine reaching its first field
-// access, which is microseconds wide. The race detector reports an
-// unsynchronised pair whenever it observes one, but it has to observe
-// one, so a single cycle can miss purely on scheduling. Repetition
-// makes the observation reliable; the collaborators are built once,
-// so the cycles themselves are cheap.
+// access, which is microseconds wide, so a single cycle can miss it
+// purely on scheduling. Repetition makes hitting it reliable; the
+// collaborators are built once, so the cycles themselves are cheap.
 const earlyStopIterations = 25
 
 // TestEarlyShutdown_NoPanicAndNoRace stops the application
@@ -38,8 +36,9 @@ const earlyStopIterations = 25
 //
 // httpServer is now built in New, on the constructing goroutine, so
 // it is written before any hook exists and can never be nil.
-// sentryEnabled is atomic. This test is what catches either one
-// coming back — under -race, which is how the suite runs.
+// sentryEnabled is atomic. This test catches the nil dereference
+// coming back, as a panic. The suite runs without Go's race
+// detector, so the data race coming back would go unreported here.
 func TestEarlyShutdown_NoPanicAndNoRace(t *testing.T) {
 	t.Parallel()