free up disk space in CI runners

[jayshrivastava]

This change frees up disk space in CI runners. See https://github.com/apache/flink/blob/02d30ace69dc18555a5085eccf70ee884e73a16e/tools/azure-pipelines/free_disk_space.sh
Before

Filesystem      Size  Used Avail Use% Mounted on
/dev/root        72G   66G  5.8G  93% /
tmpfs           7.9G   84K  7.9G   1% /dev/shm
tmpfs           3.2G  1.2M  3.2G   1% /run
tmpfs           5.0M     0  5.0M   0% /run/lock
/dev/sda16      881M   62M  758M   8% /boot
/dev/sda15      105M  6.2M   99M   6% /boot/efi
/dev/sdb1        74G  4.1G   66G   6% /mnt
tmpfs           1.6G   12K  1.6G   1% /run/user/1001

After

Filesystem      Size  Used Avail Use% Mounted on
/dev/root        72G   47G   25G  66% /
tmpfs           7.9G   84K  7.9G   1% /dev/shm
tmpfs           3.2G  1.1M  3.2G   1% /run
tmpfs           5.0M     0  5.0M   0% /run/lock
/dev/sdb16      881M   62M  758M   8% /boot
/dev/sdb15      105M  6.2M   99M   6% /boot/efi
/dev/sda1        74G  4.1G   66G   6% /mnt
tmpfs           1.6G   12K  1.6G   1% /run/user/1001

[jayshrivastava]

Btw the largest directory is 12G ./target. We can move it to /mnt instead of / if needed. For now, this change should buy us a lot of time.

[gabotechs]

👍 nice