Do not overwrite default linker flags so that LDFLAGS environment variable is considered

[traversaro]

If one wants to specify additional linked flags, those should be appended to CMAKE_SHARED_LINKER_FLAGS, not overwrite it as this would lead to the LDFLAGS environment variable to be ignored, leading to linking errors in some environments (for example conda).

For more details see:

• Do not overwrite default linker flags so that LDFLAGS environment variable is considered gazebosim/gazebo-classic#2922
• Conda Linux CI build fail due to linking problem in Wearables robotology-superbuild#681 (comment)

[diegoferigo]

Thanks @traversaro for propagating this fix to all our repositories, I'm the responsible of these linker tweaks that got spread around in the past few years 😈

Let's ignore the Website pipeline fail triggered by the push event, what's matters is the one triggered by the pull event, that succeeds. Unfortunately there's no way for push events to know against which channel (stable / nightly) to get tested, and we default with nightly (because with our development cycle it's more common). In any case, these failings happen when stable and nightly channel support different Ignition distributions like right now, and this happens quite rarely.