start-fuseki: construct classloader in a gradle-compatible manner

[NicolasRouquette]

Description

A clear and concise description of what the bug is.

the current 'startFuseki' task requires gradle <= 6.5.1 because of a subtle problem described here:

gradle/gradle#14727 (comment)

On this issue, there are some suggestions to work around this problem.

Steps to Reproduce

Steps to reproduce the behavior:

• Step 1 use gradle > 6.5.1
• Step 2 startFuseki

Expected Behavior

A clear and concise description of what should be the expected behavior.

If the bug is fixed, fuseki will start properly.
Until then, fuseki will not start because of a class not found: org/gradle/internal/classpath/Instrumented

Additional Context

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