Handle config field 'target' when calling publish (arg branch) on GitHub Actions

[eine]

From #7:

the .btd-upstream.yml file looks like this :

# Path to input sources
# Absolute or relative to the location of this configuration file
input: docs

# Path to save intermediate artifacts and products
# Absolute or relative to 'input'
output: _build

# Path to an optional requirements file
# Absolute or relative to 'input'
requirements: requirements.txt

# Domain/repo/branch|tag where sources are located:
source: upstream

# Domain/repo/branch and path where products are to be deployed:
target: gh-pages-upstream

# List of output formats
formats: [ html ]

# List of versions
#versions:
#  - id: master
#    title: ""
#    description: ""
#    warning: ""
#  - id: v0.0.0

# Images to run commands in
images:
  latex: btdi/latex

# Install theme from a repo or a tarball
theme: https://codeload.github.com/buildthedocs/sphinx.theme/tar.gz/v0`

We can see in the list line that he tries to push to gh-pages , but here i choosed gh-pages-upstream as a target in .btd-upstream.yml

[eine]

This is partially handled now. FIeld 'target' is passed as-is to the branch argument.