Support Bazel 7

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Fixes #185

---

Open this as a draft for comments and review. I will verify that the images still work.

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I had hoped the images would be equivalent, for a short-circuit verification. But they differ, so I'll take some time to do a proper functional verification.

$ bazel query --output=label 'attr(generator_function, go_image, //...)' | xargs bazel build --show_result=10
0 2>&1 | grep '^  ' | xargs sha256sum | tee after
....
$ git revert HEAD
$ env USE_BAZEL_VERSION=6.4.0 bazel query --output=label 'attr(generator_function, go_image, //...)' | xargs env USE_BAZEL_VERSION=6.4.0 bazel build --show_result=100 2>&1 | grep '^  ' | xargs sha256sum | tee before
...
$  10:52 m ██ ~/tas/pat/bb-storage diff -u ( sort -k2 before | psub) ( sort -k2 after | psub)                                          rebase
--- /tmp/.psub.80GjVgKflJ       2024-01-18 10:52:56.722977626 +0100
+++ /tmp/.psub.LBXqyL5hiO       2024-01-18 10:52:56.726977592 +0100
@@ -1,6 +1,6 @@
-f1fbbf81ac4535c2d1d596dd24e1191571372d3b772d77cc8bb89ce7401b1da7  bazel-bin/cmd/bb_copy/bb_copy_container.binary_/bb_copy_container.binary
-4e4d67a133b135c40a57dbcdda8143d9b6f4cfd0872d9818a22c79b876cc114a  bazel-bin/cmd/bb_replicator/bb_replicator_container.binary_/bb_replicator_container.binary
-b31f1c8420f078e664c7217e06192893fa9de17fe3eadd7a3b6207616fddae0d  bazel-bin/cmd/bb_storage/bb_storage_container.binary_/bb_storage_container.binary
-a6a3d6ff5e08774247653b5ee0739c6bc9dbf221436b53065f935eb0d8580ca2  bazel-out/k8-fastbuild-ST-4a519fd6d3e4/bin/cmd/bb_copy/bb_copy_container-layer.tar
-25b7e5666dc69e323e80f2028daae39b1ea10f07f728584914dd24966c4c635d  bazel-out/k8-fastbuild-ST-4a519fd6d3e4/bin/cmd/bb_replicator/bb_replicator_container-layer.tar
-4423fde8d6c5cb687d107deab4015aba43b8e582f34e185aff03569eba81c601  bazel-out/k8-fastbuild-ST-4a519fd6d3e4/bin/cmd/bb_storage/bb_storage_container-layer.tar
+60fe77c3a70db7e777ea82896c3df3a1257e8503c426f1f935450d6e58e89dbb  bazel-bin/cmd/bb_copy/bb_copy_container.binary_/bb_copy_container.binary
+fcb7dbcf11751f4e03782e5a4c633a872b4dd8f5ed7bda88b2a4e1955155e32a  bazel-bin/cmd/bb_replicator/bb_replicator_container.binary_/bb_replicator_container.binary
+c0c6db11f5263909cdca23733a0dfa82d7f836c2bb43c3ac957df9337370da45  bazel-bin/cmd/bb_storage/bb_storage_container.binary_/bb_storage_container.binary
+ea11ae10446ad898f921df00dd7bcf59b518bbd0e07dab3ee223aa9b80e07adb  bazel-out/k8-fastbuild-ST-e821f40a66f6/bin/cmd/bb_copy/bb_copy_container-layer.tar
+3c73da9a466cd02e20ac9e8f600ab5a6757398765ea25a21497eb0a1ec9b9fcb  bazel-out/k8-fastbuild-ST-e821f40a66f6/bin/cmd/bb_replicator/bb_replicator_container-layer.tar
+3111c07625320c19515ebb1e2509c99f7721eea67e60cf8b29a56f8463c7a303  bazel-out/k8-fastbuild-ST-e821f40a66f6/bin/cmd/bb_storage/bb_storage_container-layer.tar

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protobuf also complains for bazel test //...

ERROR: /root/.cache/bazel/_bazel_root/8fceff111200c246cb4996ede214e827/external/com_google_protobuf/src/google/protobuf/compiler/BUILD.bazel:140:13: @@com_google_protobuf//src/google/protobuf/compiler:protoc_aarch64_test_genrule: no such attribute ‘exec_tools’ in ‘genrule’ rule (did you mean ‘executable’?)
ERROR: /root/.cache/bazel/_bazel_root/8fceff111200c246cb4996ede214e827/external/com_google_protobuf/src/google/protobuf/compiler/BUILD.bazel:145:12: @@com_google_protobuf//src/google/protobuf/compiler:protoc_x86_64_test_genrule: no such attribute ‘exec_tools’ in ‘genrule’ rule (did you mean ‘executable’?) (edited)