gh-40955: bump brial to 1.2.15
    
this allows specifying external boost prefix, if needed


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preview.

### ⌛ Dependencies

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URL: #40955
Reported by: Dima Pasechnik
Reviewer(s): Frédéric Chapoton

Author: Release Manager

build/pkgs/brial/checksums.ini

@@ -1,4 +1,4 @@
 tarball=brial-VERSION.tar.bz2
-sha1=bfc2b2c8220bc779c8114d7ccd0094b9255f1877
-sha256=48cd95f167381699e2c538bfbf6eac35ca046dee79f797059f3879abdf5b7e66
+sha1=cc88a92ec99e280df534e1b1b1b09686a290e6f9
+sha256=1d0e7de01b1b60ad892dee6c02503f5932e2ed211f3f8dbdf1a3216c696ef201
 upstream_url=https://github.com/BRiAl/BRiAl/releases/download/VERSION/brial-VERSION.tar.bz2

build/pkgs/brial/package-version.txt

@@ -1 +1 @@
-1.2.14
+1.2.15