[Bzlmod] Using `ast-plus` toolchain leads to non reproducibility warning

[FrankPortman]

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If you use ast-plus in your toolchain, you will see something like this print out as part of the build:

DEBUG: Rule 'rules_scala++scala_deps+scala_compiler_source_2_13_16' indicated that a canonical reproducible form can be obtained by modifying arguments integrity = "sha256-6SLcL5fVRf3Ag/L88pFesAx2R/N3C5VIneC8nve1Sj4="

You can add the following in your MODULE.bazel to make the error go away:

scala_deps.compiler_srcjar(
    integrity = "sha256-6SLcL5fVRf3Ag/L88pFesAx2R/N3C5VIneC8nve1Sj4=",
    url = "https://repo1.maven.org/maven2/org/scala-lang/scala-compiler/%s/scala-compiler-%s-sources.jar" % (scala_version, scala_version),
    version = scala_version,
)

Ideally, this is not required. But if it is the correct/idiomatic solution, we should better document it.