tast: Disable cgo on building Tast binaries.

Also disable PIE because it does not work without cgo:
golang/go#30986 (comment)

Consequently, we also end up disabling ASLR for Tast binaries.
This should be okay since Tast binaries are not shipped to users.

BUG=chromium:976196
TEST=fast_build.sh -T
TEST=fast_build.sh && ~/go/bin/tast run betty example.Pass

Cq-Depend: chromium:1663975
Change-Id: I0da6cfbe45622ec874f94e74bc3b5a0bc4661508
Reviewed-on: https://chromium-review.googlesource.com/1664194
Tested-by: Shuhei Takahashi <nya@chromium.org>
Commit-Ready: Shuhei Takahashi <nya@chromium.org>
Legacy-Commit-Queue: Commit Bot <commit-bot@chromium.org>
Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>

fast_build.sh

@@ -31,6 +31,12 @@ readonly TAST_OUT="${GOHOME}/bin/tast"
 # be emerged beforehand.
 export GOPATH="$(IFS=:; echo "${SRCDIRS[*]}"):/usr/lib/gopath"
 
+# Disable cgo and PIE on building Tast binaries. See:
+# https://crbug.com/976196
+# https://github.com/golang/go/issues/30986#issuecomment-475626018
+export CGO_ENABLED=0
+export GOPIE=0
+
 readonly CMD=$(basename "${0}")
 
 # Prints usage information and exits.

src/chromiumos/cmd/tast/build/build.go

@@ -79,7 +79,13 @@ func Build(ctx context.Context, cfg *Config, pkg, outDir, stageName string) (out
 
 	const ldFlags = "-ldflags=-s -w"
 
-	env := append(os.Environ(), "GOPATH="+strings.Join(cfg.Workspaces, ":"))
+	env := append(os.Environ(),
+		"GOPATH="+strings.Join(cfg.Workspaces, ":"),
+		// Disable cgo and PIE on building Tast binaries. See:
+		// https://crbug.com/976196
+		// https://github.com/golang/go/issues/30986#issuecomment-475626018
+		"CGO_ENABLED=0",
+		"GOPIE=0")
 
 	// This is frustrating:
 	//