[3.0] CI: fix the path of SDK installed on old-macmini (macmini-mojave (2012))

This path seems to be barely usable:

xcrun --show-sdk-version
xcodebuild: error: SDK "/Applications/Xcode9.2.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.13.sdk" cannot be located.
xcrun: error: unable to lookup item 'SDKVersion' in SDK '/Applications/Xcode9.2.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.13.sdk'

This is also reflected by xcodebuild:

    ARCHS = x86_64
    CLANG_CXX_LIBRARY = libc++
    MACOSX_DEPLOYMENT_TARGET = 10.7
    OTHER_CFLAGS = -fno-stack-check
    SDKROOT = macosx10.15
    WARNING_CFLAGS = -Werror=partial-availability

The default XCode use by xcrun is not compatible with the 10.13 SDK. Forcing the DEVELOPER_DIR environment variable fixes this issue.

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Plus show the SDK version used for each build.