Upgrade conan in conan package jobs

The minimum version of conan we can support can change depending on
what our dependencies require, so there's no point in testing our
conan builds against a particular version of conan.  Therefore we've
decided to just always build against the latest version of conan.

.ci/gitlab-ci.yml

@@ -290,7 +290,7 @@ conan-linux-gcc7:
   needs: [check-format]
   image: $DOCKER_REGISTRY/rewriting/gtirb/ubuntu18
   script: &conan-build
-    - python3 -m pip install conan
+    - python3 -m pip install --upgrade conan
     - conan profile new default --detect
     - conan profile update settings.compiler.libcxx=libstdc++11 default
     - export PYTHONPATH="$(pwd)"
@@ -311,7 +311,7 @@ conan-linux-gcc9:
     - systeminfo
     - export PATH="C:\\Program Files\\Python38;$PATH"
     # Install conan
-    - python -m pip install conan
+    - python -m pip install --upgrade conan
     # Run build script
     - export PYTHONPATH="$(cygpath -w $(pwd))"
     - export PATH="$PATH;C:\\Program Files\\CMake\\bin"
@@ -340,7 +340,7 @@ conan-windows-32:
     - systeminfo
     - export PATH="C:\\Program Files\\Python38;$PATH"
     # Install conan
-    - python -m pip install conan
+    - python -m pip install --upgrade conan
     # Run build script
     - export PYTHONPATH="$(cygpath -w $(pwd))"
     - export PATH="$PATH;C:\\Program Files\\CMake\\bin"