Add support for nightly Swift 5.5 Docker images

[shahmishal]

No description provided.

[shahmishal]

@swift-ci test

[shahmishal]

  | PASSED_nightly-5.5_amazonlinux_2_slim.log | Apr 21, 2021 9:03:10 PM | 8.87 KB | view
  | PASSED_nightly-5.5_amazonlinux_2.log | Apr 21, 2021 9:08:04 PM | 60.06 KB | view
  | PASSED_nightly-5.5_centos_7_slim.log | Apr 21, 2021 8:54:45 PM | 4.65 KB | view
  | PASSED_nightly-5.5_centos_7.log | Apr 21, 2021 9:00:51 PM | 54.25 KB | view
  | PASSED_nightly-5.5_centos_8_slim.log | Apr 21, 2021 8:48:21 PM | 4.67 KB | view
  | PASSED_nightly-5.5_centos_8.log | Apr 21, 2021 8:52:42 PM | 42.59 KB | view
  | PASSED_nightly-5.5_ubuntu_16.04_slim.log | Apr 21, 2021 8:42:49 PM | 27.79 KB | view
  | PASSED_nightly-5.5_ubuntu_16.04.log | Apr 21, 2021 8:46:37 PM | 58.86 KB | view
  | PASSED_nightly-5.5_ubuntu_18.04_slim.log | Apr 21, 2021 8:36:29 PM | 31.24 KB | view
  | PASSED_nightly-5.5_ubuntu_18.04.log | Apr 21, 2021 8:40:20 PM | 65.43 KB | view
  | PASSED_nightly-5.5_ubuntu_20.04_slim.log | Apr 21, 2021 8:30:15 PM | 31.70 KB | view
  | PASSED_nightly-5.5_ubuntu_20.04.log | Apr 21, 2021 8:33:52 PM | 56.54 KB | view