Fix the bug introduced to image_tag (#137)

Not sure how I missed this, but I had excluded the pipe to file. Was
able to test pushing locally in orion and it works now.

image/defs.bzl

@@ -38,7 +38,7 @@ def image_tag(name, tag, extra_tags = []):
     native.genrule(
         name = name,
         outs = ["{}.txt".format(name)],
-        cmd = "echo {} && echo {} > $@".format(tag, "\n".join(extra_tags)),
+        cmd = "echo {} > $@ && echo {} >> $@".format(tag, "\n".join(extra_tags)),
         tags = ["manual"],
     )
 
@@ -55,10 +55,11 @@ def image_stamp_tag(name, var, extra_tags = []):
         name = name,
         outs = ["{}.txt".format(name)],
         # `(?<=A)B` in regex is a positive lookbehind - finds expression B that's preceded with A
-        cmd = "cat bazel-out/stable-status.txt | grep -Po '(?<={}\\s).*' && echo {} > $@".format(
+        cmd = (
+            "cat bazel-out/stable-status.txt | grep -Po '(?<={}\\s).*' > $@ && echo {} >> $@".format(
             var,
             "\n".join(extra_tags),
-        ),
+        )),
         stamp = True,
         tags = ["manual"],
     )