fix build-test after breaking nightly proc-macro change

[syphar]

This fixes the build-test breaking on master.

Reason is rust-lang/rust#139671, fixed in dtolnay/proc-macro2#497, released in proc-macro2 1.0.95.

The thing I'm not 100% certain is: we added this test in #1800, and I don't know if the new proc-macro2 version also covers what we wanted to test here.

full extracted error from the CI logs

[stderr]  Documenting proc-macro2 v1.0.33 (/opt/rustwide/workdir)    
[stderr] error[E0412]: cannot find type `SourceFile` in crate `proc_macro`    
[stderr]    --> src/wrapper.rs:362:26    
[stderr]     |    
[stderr] 362 |     Compiler(proc_macro::SourceFile),    
[stderr]     |                          ^^^^^^^^^^ not found in `proc_macro`    
[stderr]     |    
[stderr] help: consider importing one of these structs    
[stderr]     |    
[stderr] 1   + use crate::SourceFile;    
[stderr]     |    
[stderr] 1   + use crate::fallback::SourceFile;    
[stderr]     |    
[stderr] help: if you import `SourceFile`, refer to it directly    
[stderr]     |    
[stderr] 362 -     Compiler(proc_macro::SourceFile),    
[stderr] 362 +     Compiler(SourceFile),    
[stderr]     |    
[stderr]     
[stderr] error[E0412]: cannot find type `SourceFile` in crate `proc_macro`    
[stderr]    --> src/wrapper.rs:368:32    
[stderr]     |    
[stderr] 368 |     fn nightly(sf: proc_macro::SourceFile) -> Self {    
[stderr]     |                                ^^^^^^^^^^ not found in `proc_macro`    
[stderr]     |    
[stderr] help: consider importing one of these structs    
[stderr]     |    
[stderr] 1   + use crate::SourceFile;    
[stderr]     |    
[stderr] 1   + use crate::fallback::SourceFile;    
[stderr]     |    
[stderr] help: if you import `SourceFile`, refer to it directly    
[stderr]     |    
[stderr] 368 -     fn nightly(sf: proc_macro::SourceFile) -> Self {    
[stderr] 368 +     fn nightly(sf: SourceFile) -> Self {    

[GuillaumeGomez]

Thanks!