Override MILL_VERSION to ensure we get a portable version of mill

[jmarshall]

By default hail/mill downloads mill-dist-native-linux-amd64-1.0.4.exe, which as built requires at least GLIBC_2.34. Bullseye provides 2.31 only. We might prefer to update Dockerfile.base to bookworm or trixie, but they do not provide openjdk-11 which Hail still insists on. (Upstream's similar images use Ubuntu Noble, but that has the disadvantage for us of using Python 3.12 rather than 3.11.)

This resolves the previous driver image build failure while building Hail:

#6 30.95 Downloading mill 1.0.4 from https://repo1.maven.org/maven2/com/lihaoyi/mill-dist-native-linux-amd64/1.0.4/mill-dist-native-linux-amd64-1.0.4.exe ...
[…]
#6 31.25 /root/.cache/mill/download/1.0.4-native-linux-amd64: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.32' not found (required by /root/.cache/mill/download/1.0.4-native-linux-amd64)
#6 31.25 /root/.cache/mill/download/1.0.4-native-linux-amd64: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.34' not found (required by /root/.cache/mill/download/1.0.4-native-linux-amd64)
#6 31.25 make: *** [Makefile:85: out/hail/2.12/assembly.dest/out.jar] Error 1

Thus working around com-lihaoyi/mill#5568.