gitian: Limit sourced script to just assignments

Previously, the sourced script would create the source tarball. Now, it
only assigns variables and the source-ing script has more flexibility in
determining what to do with these variables.

See later commit showing how this flexibility is useful in our Guix
builds.

contrib/gitian-descriptors/assign_DISTNAME

@@ -0,0 +1,12 @@
+# Copyright (c) 2020 The Bitcoin Core developers
+# Distributed under the MIT software license, see the accompanying
+# file COPYING or http://www.opensource.org/licenses/mit-license.php.
+#
+# A helper script to be sourced into the gitian descriptors
+
+if RECENT_TAG="$(git describe --exact-match HEAD)"; then
+    VERSION="${RECENT_TAG#v}"
+else
+    VERSION="$(git rev-parse --short HEAD)"
+fi
+DISTNAME="bitcoin-${VERSION}"

contrib/gitian-descriptors/gitian-linux.yml

@@ -140,9 +140,15 @@ script: |
   create_per-host_faketime_wrappers "${REFERENCE_DATETIME}"
   export PATH=${WRAP_DIR}:${PATH}
 
-  # Create the git archive, and define DISTNAME and GIT_ARCHIVE variables.
-  # shellcheck source=contrib/gitian-descriptors/make_git_archive
-  source contrib/gitian-descriptors/make_git_archive
+  # Define DISTNAME variable.
+  # shellcheck source=contrib/gitian-descriptors/assign_DISTNAME
+  source contrib/gitian-descriptors/assign_DISTNAME
+
+  GIT_ARCHIVE="${OUTDIR}/src/${DISTNAME}.tar.gz"
+
+  # Create the source tarball
+  mkdir -p "$(dirname "$GIT_ARCHIVE")"
+  git archive --output="$GIT_ARCHIVE" HEAD
 
   ORIGPATH="$PATH"
   # Extract the git archive into a dir for each host and build

contrib/gitian-descriptors/gitian-osx.yml

@@ -103,9 +103,15 @@ script: |
   create_per-host_faketime_wrappers "${REFERENCE_DATETIME}"
   export PATH=${WRAP_DIR}:${PATH}
 
-  # Create the git archive, and define DISTNAME and GIT_ARCHIVE variables.
-  # shellcheck source=contrib/gitian-descriptors/make_git_archive
-  source contrib/gitian-descriptors/make_git_archive
+  # Define DISTNAME variable.
+  # shellcheck source=contrib/gitian-descriptors/assign_DISTNAME
+  source contrib/gitian-descriptors/assign_DISTNAME
+
+  GIT_ARCHIVE="${OUTDIR}/src/${DISTNAME}.tar.gz"
+
+  # Create the source tarball
+  mkdir -p "$(dirname "$GIT_ARCHIVE")"
+  git archive --output="$GIT_ARCHIVE" HEAD
 
   ORIGPATH="$PATH"
   # Extract the git archive into a dir for each host and build

contrib/gitian-descriptors/gitian-win.yml

@@ -108,9 +108,15 @@ script: |
   create_per-host_compiler_wrapper "${REFERENCE_DATETIME}"
   export PATH=${WRAP_DIR}:${PATH}
 
-  # Create the git archive, and define DISTNAME and GIT_ARCHIVE variables.
-  # shellcheck source=contrib/gitian-descriptors/make_git_archive
-  source contrib/gitian-descriptors/make_git_archive
+  # Define DISTNAME variable.
+  # shellcheck source=contrib/gitian-descriptors/assign_DISTNAME
+  source contrib/gitian-descriptors/assign_DISTNAME
+
+  GIT_ARCHIVE="${OUTDIR}/src/${DISTNAME}.tar.gz"
+
+  # Create the source tarball
+  mkdir -p "$(dirname "$GIT_ARCHIVE")"
+  git archive --output="$GIT_ARCHIVE" HEAD
 
   ORIGPATH="$PATH"
   # Extract the git archive into a dir for each host and build