Disable MSYS2's path munging for `stringreplace` (#46803)
This was causing our rewriting of the loader's RPATH emulation to fail
after running `make install`, as MSYS2 was rewriting our list that looks
like:
```
../bin/libgcc_s_seh-1.dll:../bin/libopenlibm.dll:@../bin/libjulia-internal.dll:@../bin/libjulia-codegen.dll:
```
Into one that looked like:
```
..\bin\libgcc_s_seh-1.dll;..\bin\libopenlibm.dll;@..\bin\libjulia-internal.dll;@..\bin\libjulia-codegen.dll;
```
By exporting `MSYS2_ARG_CONV_EXCL='*'` for all `stringreplace`
invocations, we dodge this issue, as documented by MSYS2 [0].
[0] https://www.msys2.org/wiki/Porting/#filesystem-namespaces
