CI: Switch to available latest images
`mirror.gcr.io/library/busybox:1.32.0` is unavailable.
```console
$ curl -s https://mirror.gcr.io//v2/library/busybox/tags/list | jq '.tags'
[
"1.26.2",
"1.27.2",
"1.28",
"1.29",
"1.29.2",
"1.29.3",
"1.30",
"1.30.1",
"1.31",
"1.31.0",
"1.31.1",
"1.32",
"latest"
]
```
`mirror.gcr.io/library/alpine:3.13` is unavailable.
```console
$ curl -s https://mirror.gcr.io//v2/library/alpine/tags/list | jq '.tags'
[
"20200428",
"3",
"3.10",
"3.11",
"3.12",
"3.12.0",
"3.12.1",
"3.13.5",
"3.3",
"3.4",
"3.6",
"3.7",
"3.8",
"3.9",
"latest"
]
```
Signed-off-by: Kohei Tokunaga <ktokunaga.mail@gmail.com>
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