When I added the Coverity workflow in a56b623 (ci: add a GitHub
workflow to submit Coverity scans, 2023-09-25), I merely converted an
Azure Pipeline definition that had been running successfully for ages.

In the meantime, the current Coverity documentation describes a very
different way to install the analysis tool, recommending to add the
`bin/` directory to the _end_ of `PATH` (when originally, IIRC, it was
recommended to add it to the _beginning_ of the `PATH`).

This is crucial! The reason is that the current incarnation of the
Windows variant of Coverity's analysis tools come with a _lot_ of DLL
files in their `bin/` directory, some of them interferring rather badly
with the `gcc.exe` in Git for Windows' SDK that we use to run the
Coverity build. The symptom is a cryptic error message:

  make: *** [Makefile:2960: headless-git.o] Error 1
  make: *** Waiting for unfinished jobs....
  D:\git-sdk-64-minimal\mingw64\bin\windres.exe: preprocessing failed.
  make: *** [Makefile:2679: git.res] Error 1
  make: *** [Makefile:2893: git.o] Error 1
  make: *** [Makefile:2893: builtin/add.o] Error 1
  Attempting to detect unconfigured compilers in build
  |0----------25-----------50----------75---------100|
  ****************************************************
  Warning:  Build command make.exe exited with code 2. Please verify that the build completed successfully.
  Warning:  Emitted 0 C/C++ compilation units (0%) successfully

  0 C/C++ compilation units (0%) are ready for analysis
   For more details, please look at:
      D:/a/git/git/cov-int/build-log.txt

The log (which the workflow is currently not configured to reveal) then
points out that the `windows.h` header cannot be found, which is _still_
not very helpful. The underlying root cause is that the `gcc.exe` in Git
for Windows' SDK determines the location of the header files via the
location of certain DLL files, and finding the "wrong" ones first on the
`PATH` misleads that logic.

Let's fix this problem by following Coverity's current recommendation
and append the `bin/` directory in which `cov-int` can be found to the
_end_ of `PATH`.

Signed-off-by: Johannes Schindelin <[email protected]>