[bootstrap] Force-disable CGO when building fint

Apparently if a system GCC is found then Go will enable it, so when we
added GCC back into the GCE images (but didn't include the rest of the
system headers), this Go build failed.

The rest of our Go builds are fine because our Go toolchain disables CGO
like this, but this particular script has to run outside of GN/ninja.

Bug: 127321
Change-Id: I89cfe2da048f44a52259efbe2991f2fdf4640e16
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/855107
Fuchsia-Auto-Submit: Drew Fisher <zarvox@google.com>
Reviewed-by: Oliver Newman <olivernewman@google.com>
Reviewed-by: Nathan Mulcahey <nmulcahey@google.com>
Commit-Queue: Drew Fisher <zarvox@google.com>
(cherry picked from commit ccccb28f2d2b6f56fa036de96f51a998b7a2f193)
diff --git a/tools/integration/bootstrap.sh b/tools/integration/bootstrap.sh
index 6bddba2..64443cc 100755
--- a/tools/integration/bootstrap.sh
+++ b/tools/integration/bootstrap.sh
@@ -87,7 +87,7 @@
 
   GOROOT_DIR="$FUCHSIA_ROOT/prebuilt/third_party/go/$(host_platform)"
   readonly go_bin="$GOROOT_DIR/bin/go"
-  GOCACHE="$GOCACHE_DIR" GOROOT="$GOROOT_DIR" GOPROXY=off $go_bin build \
+  GOCACHE="$GOCACHE_DIR" GOROOT="$GOROOT_DIR" GOPROXY=off CGO_ENABLED=0 $go_bin build \
     -o "$output" ./tools/integration/fint/cmd/fint
 }