os: build migrations in parallel

[tjkirch]

Description of changes:

This took os build time from 388 seconds to 218 seconds on my machine, almost 3 minutes saved.

Testing done:

The built RPM still has all of the migrations, same as before the change:

./aarch64-bottlerocket-linux-gnu/sys-root/usr/share/migrations/migrate_v0.3.2_admin-container-v0-5-0
./aarch64-bottlerocket-linux-gnu/sys-root/usr/share/migrations/migrate_v0.4.1_add-version-lock-ignore-waves
./aarch64-bottlerocket-linux-gnu/sys-root/usr/share/migrations/migrate_v0.4.1_pivot-repo-2020-07-07
./aarch64-bottlerocket-linux-gnu/sys-root/usr/share/migrations/migrate_v0.5.0_add-cluster-domain
./aarch64-bottlerocket-linux-gnu/sys-root/usr/share/migrations/migrate_v0.5.0_admin-container-v0-5-2
./aarch64-bottlerocket-linux-gnu/sys-root/usr/share/migrations/migrate_v0.5.0_control-container-v0-4-1
./aarch64-bottlerocket-linux-gnu/sys-root/usr/share/migrations/migrate_v1.0.0_ecr-helper-admin
./aarch64-bottlerocket-linux-gnu/sys-root/usr/share/migrations/migrate_v1.0.0_ecr-helper-control
./aarch64-bottlerocket-linux-gnu/sys-root/usr/share/migrations/migrate_v1.0.2_add-enable-spot-instance-draining
./aarch64-bottlerocket-linux-gnu/sys-root/usr/share/migrations/migrate_v1.0.3_add-sysctl

This is the command run during the build:

#17 0.167 + /usr/bin/cargo build --offline --locked --verbose --target aarch64-bottlerocket-linux-musl --release --manifest-path /home/builder/rpmbuild/BUILD/sources/Cargo.toml -p apiclient -p migrate-admin-container-v0-5-0 -p pivot-repo-2020-07-07 -p add-version-lock-ignore-waves -p add-cluster-domain -p migrate-admin-container-v0-5-2 -p migrate-control-container-v0-4-1 -p ecr-helper-control -p ecr-helper-admin -p add-enable-spot-instance-draining -p add-sysctl

Terms of contribution:

By submitting this pull request, I agree that this contribution is dual-licensed under the terms of both the Apache License, version 2.0, and the MIT license.

[etungsten]

🚀