CI: Maintenance

- Rename .travis dir to .ci, since it isn't just for Travis
- Convert Linux build scripts to posix sh
- Clean up some scripts per shellcheck

.ci/travis.env

@@ -0,0 +1,16 @@
+# Variables set by Travis CI
+TRAVIS_PULL_REQUEST
+TRAVIS_BRANCH
+TRAVIS_COMMIT
+# Variables set by Azure Pipelines
+IS_AZURE
+BUILD_REASON
+BUILD_SOURCEVERSION
+BUILD_ARTIFACTSTAGINGDIRECTORY
+BUILD_REPOSITORY_NAME
+BUILD_SOURCEBRANCHNAME
+# Variables for Travis build matrix
+COMPILER
+DEPLOY_APPIMAGE
+# Private Travis CI variables
+GITHUB_TOKEN