Fix wrong exists-condition and echo when downloading

The conditions were inverted due to combination via `&&`.
Use plain `if` to make it easier to read and echo when downloading anything.
Also reformat to make it easier to read and append the extension to the script at the start to avoid forgetting it.

ci/drone/functions.star

@@ -12,36 +12,37 @@
 def download_from_boostCI(filename, boostCI_dir, out_dir=None):
   if out_dir == None:
     out_dir = boostCI_dir
-  return 'curl -s -S --retry 10 --create-dirs -L "https://github.com/boostorg/boost-ci/raw/master/{1}/{0}" -o "{2}/{0}" && chmod 755 {2}/{0}'.format(filename, boostCI_dir, out_dir)
+  url = 'https://github.com/boostorg/boost-ci/raw/master/%s/%s' % (boostCI_dir, filename)
+  target_path = '%s/%s' & (out_dir, filename)
+  return 'echo "Downloading {0} to {1}"; curl -s -S --retry 10 --create-dirs -L "{0}" -o "{1}" && chmod 755 {1}'.format(url, target_path)
 
 # Common steps for unix systems
 # Takes the install script (inside the Boost.CI "ci/drone" folder) and the build script (relative to the root .drone folder)
 def unix_common(install_script, buildscript_to_run):
-  # Commands to download scripts
-  # Factored out for ease of writing the commands
-  download_scripts = [
-    # Only when not testing Boost.CI
-    'if [ "$(basename "$DRONE_REPO")" != "boost-ci" ]; then',
-    # Install script
-    download_from_boostCI(install_script, 'ci/drone'),
-    # Default build script (if not exists) 
-    '[ ! -e .drone/drone.sh ] || ' + download_from_boostCI('drone.sh', '.drone'),
-    # Chosen build script inside .drone (if a filename/stem and does not exist)
-    '{{ [ "$(basename "{0}")" = "{0}" ] && [ ! -e .drone/{0}.sh ]; }} || '.format(buildscript_to_run) + download_from_boostCI(buildscript_to_run + ".sh", '.drone'),
-    # Done
-    'fi',
-  ]
+  if not buildscript_to_run.endswith('.sh')
+    buildscript_to_run += '.sh'
   return [
     "echo '==================================> SETUP'",
     "uname -a",
     "export PATH=/usr/local/bin:$PATH",
-    '\n'.join(download_scripts),
+    '\n'.join([
+      # Only when not testing Boost.CI
+      'if [ "$(basename "$DRONE_REPO")" != "boost-ci" ]; then',
+        # Install script
+        download_from_boostCI(install_script, 'ci/drone'),
+        # Default build script (if not exists) 
+        'if [ ! -e .drone/drone.sh ]; then %s; fi' % download_from_boostCI('drone.sh', '.drone'),
+        # Chosen build script inside .drone (if a filename and does not exist)
+        'if [ "$(basename "{0}")" = "{0}" ] && [ ! -e .drone/{0} ]; then {1}; fi'.format(buildscript_to_run, download_from_boostCI(buildscript_to_run, '.drone')),
+      # Done
+      'fi',
+    ]),
 
     "echo '==================================> PACKAGES'",
     "ci/drone/" + install_script,
 
     "echo '==================================> INSTALL AND TEST'",
-    ".drone/%s.sh" % buildscript_to_run,
+    ".drone/" + buildscript_to_run,
   ]
 
 # Generate pipeline for Linux platform compilers.