Merge #15609: scripts and tools: Set 'distro' explicitly

b8705a091565d4373e0e8ad4aa764cfd906708db Set 'distro' explicitly (Hennadii Stepanov)

Pull request description:

  The [gitian-builder](https://github.com/devrandom/gitian-builder) implicitly uses `ubuntu` as a default distro.

  [bin/gbuild#L237](81edd2fc8e/bin/gbuild (L237)):
  ```ruby
  distro = build_desc["distro"] || "ubuntu"
  ```

  This PR sets a gitian building distro explicitly in description files.

Tree-SHA512: d2a692047f3466a5c637433610854d1100fe68a78fb03b4a81c70911fa14b0228d9cf25bcf115930aab9cc8c5063dacaf79bcd151f1c3f6fee6419389aefcb8b

contrib/gitian-descriptors/gitian-linux.yml

@@ -1,6 +1,7 @@
 ---
 name: "bitcoin-core-linux-0.19"
 enable_cache: true
+distro: "ubuntu"
 suites:
 - "bionic"
 architectures:

contrib/gitian-descriptors/gitian-osx-signer.yml

@@ -1,5 +1,6 @@
 ---
 name: "bitcoin-dmg-signer"
+distro: "ubuntu"
 suites:
 - "bionic"
 architectures:

contrib/gitian-descriptors/gitian-osx.yml

@@ -1,6 +1,7 @@
 ---
 name: "bitcoin-core-osx-0.19"
 enable_cache: true
+distro: "ubuntu"
 suites:
 - "bionic"
 architectures:

contrib/gitian-descriptors/gitian-win-signer.yml

@@ -1,5 +1,6 @@
 ---
 name: "bitcoin-win-signer"
+distro: "ubuntu"
 suites:
 - "bionic"
 architectures:

contrib/gitian-descriptors/gitian-win.yml

@@ -1,6 +1,7 @@
 ---
 name: "bitcoin-core-win-0.19"
 enable_cache: true
+distro: "ubuntu"
 suites:
 - "bionic"
 architectures: